gitlab-ci: Make use of 'before_script' less unorthodox

Since !1171 the 'before_script' was being used by me for organization, but
this is at least unexpected to "mainstream" .yml pratice and can be wrong.
For example, the universal variables can't be properly used this way.
So, let's use 'before_script' only for the (official) purpose: save code.

.gitlab-ci.yml

@@ -46,7 +46,7 @@ stages:
     key: $CI_JOB_NAME
     paths:
       - _ccache/
-  # Universal variables (works in all OSes and archs)
+  # Universal variables (works in all POSIX OSes and archs)
   before_script:
     - export PATH="$GIMP_PREFIX/bin:$PATH"
     - gcc -print-multi-os-directory | grep . && LIB_DIR=$(gcc -print-multi-os-directory | sed 's/\.\.\///g') || LIB_DIR="lib"
@@ -180,7 +180,8 @@ image-debian-x64:
   needs: ["image-debian-x64"]
   stage: dependencies
   image: $CI_REGISTRY_IMAGE:build-debian-latest
-  before_script:
+  script:
+    # Clone babl and GEGL
     - if [ "$CI_COMMIT_TAG" != "" ]; then
         repo=https://gitlab.gnome.org/GNOME/babl.git;
         babl_tag=$(git ls-remote --tags --exit-code --refs "$repo" |
@@ -196,9 +197,7 @@ image-debian-x64:
       fi
     - git clone $babl_branch --depth=${GIT_DEPTH} https://gitlab.gnome.org/GNOME/babl.git _babl
     - git clone $gegl_branch --depth=${GIT_DEPTH} https://gitlab.gnome.org/GNOME/gegl.git _gegl
-    # Universal variables
-    - !reference [.default, before_script]
-  script:
+    # Build babl and GEGL
     - mkdir _babl/_build${ARTIFACTS_SUFFIX} && cd _babl/_build${ARTIFACTS_SUFFIX}
     - meson setup .. -Dprefix="${GIMP_PREFIX}"
     - ninja
@@ -359,7 +358,7 @@ packaging-flatpak-x64:
       paths:
         - '.flatpak-builder/cache'
         - '.flatpak-builder/ccache'
-  before_script:
+  script:
     # Report the installed versions of the runtime
     - flatpak info org.gnome.Platform
     - flatpak info org.gnome.Sdk
@@ -376,7 +375,7 @@ packaging-flatpak-x64:
 
     # GNOME script to customize the manifest
     - rewrite-flatpak-manifest ${MANIFEST_PATH} ${FLATPAK_MODULE} ${CONFIG_OPTS}
-  script:
+
     # Build deps and GIMP
     # (Our script is too long for Gitlab and no complete output would be collected
     # To make debugging actually possible, let's save logs as a file.)
@@ -416,6 +415,8 @@ packaging-flatpak-x64:
     key: $CI_JOB_NAME
     paths:
       - .cache/crossroad/
+  # TODO: Crossroads handling of universal variables still not perfect
+  before_script: []
 
 .win-x64-cross:
   rules:
@@ -433,9 +434,8 @@ deps-win-x64-cross:
   needs: ["image-debian-x64"]
   stage: dependencies
   image: $CI_REGISTRY_IMAGE:build-debian-latest
-  before_script:
-    - bash -x build/windows/gitlab-ci/1_build-deps-crossroad.sh
   script:
+    - bash -x build/windows/gitlab-ci/1_build-deps-crossroad.sh
     - crossroad w64 gimp --run="build/windows/gitlab-ci/1_build-deps-crossroad.sh"
   artifacts:
     paths:
@@ -451,9 +451,8 @@ gimp-win-x64-cross:
   needs: ["deps-win-x64-cross", "gimp-debian-x64"]
   stage: gimp
   image: $CI_REGISTRY_IMAGE:build-debian-latest
-  before_script:
-    - bash -x build/windows/gitlab-ci/2_build-gimp-crossroad.sh
   script:
+    - bash -x build/windows/gitlab-ci/2_build-gimp-crossroad.sh
     - crossroad w64 gimp --run="build/windows/gitlab-ci/2_build-gimp-crossroad.sh"
   artifacts:
     paths:
@@ -470,7 +469,7 @@ packaging-win-x64-nightly:
   variables:
     CROSSROAD_PLATFORM: "w64"
   cache: []
-  before_script:
+  script:
     - apt-get update
     - apt-get install -y --no-install-recommends
                          binutils
@@ -478,7 +477,6 @@ packaging-win-x64-nightly:
                          file
                          libglib2.0-bin
                          python3
-  script:
     - bash -x build/windows/gitlab-ci/3_package-gimp-uni_base.sh
     - cd gimp-x64
     - bash -x ../build/windows/gitlab-ci/3_package-gimp-uni_sym.sh
@@ -503,6 +501,8 @@ packaging-win-x64-nightly:
   variables:
     BUILD_TYPE: "CI_NATIVE"
     CHERE_INVOKING: "yes"
+  # We use the same universal variables but not directly from PS
+  before_script: []
 
 .win-a64:
   extends: .win
@@ -808,11 +808,10 @@ clang-format:
     - merge_requests
   needs: []
   stage: analysis
-  before_script:
+  script:
     - apt-get update
     - apt-get install -y clang-format
                          git
-  script:
     - .gitlab/run_style_check_diff.sh
   allow_failure: true
   artifacts:
@@ -828,10 +827,9 @@ cppcheck:
   needs: []
   stage: analysis
   cache: []
-  before_script:
+  script:
     - apt-get update
     - apt-get install -y cppcheck
-  script:
     - cppcheck -q -j8 --enable=all --force --output-file=cppcheck.xml --xml --xml-version=2
         -i _build-x64 -i _build-x86 -i _deps -i _install-x64 -i _install-x86 -i .local -i .cache .
     - mkdir report