gitlab-ci, build: Drop custom "CI_*" variables

Let's figure out what job is running using the predefined GitLab variables.

.gitlab-ci.yml

@@ -390,7 +390,6 @@ gimp-flatpak-x64:
     - if: '$CI_PIPELINE_SOURCE == "push" && $CI_OPEN_MERGE_REQUESTS == null && $CI_COMMIT_TAG == null'
   image: $CI_REGISTRY_IMAGE:build-debian-latest
   variables:
-    BUILD_TYPE: "CI_CROSS"
     # Needed because by default, Debian has now changed
     # "sysconfig.get_default_scheme()" from "posix_prefix" to "posix_local" which
     # adds a local/ folder to the install prefix of setup.py. This environment
@@ -452,7 +451,6 @@ gimp-win-x64-cross:
     # Merge requests with appropriate label.
     - if: '$CI_MERGE_REQUEST_LABELS =~ /.*5. Windows Installer.*/'
   variables:
-    BUILD_TYPE: "CI_NATIVE"
     CHERE_INVOKING: "yes"
   # We use the same universal variables but not directly from PS
   before_script: []

build/windows/gitlab-ci/1_build-deps-msys2.sh

@@ -12,9 +12,7 @@ else # [[ "$MSYSTEM_CARCH" == "i686" ]];
   export ARTIFACTS_SUFFIX="-x86"
 fi
 
-if [[ "$BUILD_TYPE" == "CI_NATIVE" ]]; then
-  GIMP_DIR=""
-else
+if [[ -z "$GITLAB_CI" ]]; then
   # Make the script work locally
   if [[ "$0" != "build/windows/gitlab-ci/1_build-deps-msys2.sh" ]]; then
     echo "To run this script locally, please do it from to the gimp git folder"

build/windows/gitlab-ci/2_build-gimp-msys2.sh

@@ -15,7 +15,7 @@ else # [[ "$MSYSTEM_CARCH" == "i686" ]];
   export MSYS2_PREFIX="c:/msys64${MSYSTEM_PREFIX}"
 fi
 
-if [[ "$BUILD_TYPE" == "CI_NATIVE" ]]; then
+if [[ "$GITLAB_CI" ]]; then
   # XXX We've got a weird error when the prefix is in the current dir.
   # Until we figure it out, this trick seems to work, even though it's
   # completely ridiculous.
@@ -125,7 +125,7 @@ make_cmd ()
   echo "Please run the gimp.cmd file to get proper plug-in support."> ${GIMP_PREFIX}/README.txt
 }
 
-if [[ "$BUILD_TYPE" == "CI_NATIVE" ]]; then
+if [[ "$GITLAB_CI" ]]; then
   make_cmd CI %cd%
 
   cd ..

build/windows/gitlab-ci/3_bundle-gimp-uni_base.sh

@@ -19,7 +19,7 @@ else # [[ "$MSYSTEM_CARCH" == "i686" ]];
 fi
 
 
-if [[ "$BUILD_TYPE" == "CI_CROSS" ]]; then
+if [[ "$CI_JOB_NAME" =~ "cross" ]]; then
   apt-get update
   apt-get install -y --no-install-recommends   \
                      binutils                  \
@@ -32,7 +32,7 @@ fi
 
 # Bundle deps and GIMP files
 export GIMP_PREFIX="`realpath ./_install`${ARTIFACTS_SUFFIX}"
-if [[ "$BUILD_TYPE" == "CI_CROSS" ]]; then
+if [[ "$CI_JOB_NAME" =~ "cross" ]]; then
   export GIMP_PREFIX="`realpath ./_install`${ARTIFACTS_SUFFIX}-cross"
   export MSYS_PREFIX="$GIMP_PREFIX"
 fi
@@ -142,12 +142,12 @@ done
 ### .pdb (CodeView) debug symbols
 ### TODO: REMOVE 'if [[ "$MSYSTEM...' WHEN GCC 14 IS ON MSYS2
 ### crossroad don't have LLVM/Clang backend yet
-if [[ "$MSYSTEM_CARCH" != "i686" ]] && [[ "$BUILD_TYPE" != "CI_CROSS" ]]; then
+if [[ "$MSYSTEM_CARCH" != "i686" ]] && [[ ! "$CI_JOB_NAME" =~ "cross" ]]; then
   cp -fr ${GIMP_PREFIX}/bin/*.pdb ${GIMP_DISTRIB}/bin/
 fi
 
 ## Optional executables, .DLLs and resources for GObject Introspection support
-if [[ "$BUILD_TYPE" != "CI_CROSS" ]]; then
+if [[ ! "$CI_JOB_NAME" =~ "cross" ]]; then
   cp -fr ${MSYS_PREFIX}/bin/libgirepository-*.dll ${GIMP_DISTRIB}/bin/
   python3 build/windows/gitlab-ci/3_bundle-gimp-uni_dep.py ${GIMP_DISTRIB}/bin/libgirepository-*.dll ${GIMP_PREFIX}/ ${MSYS_PREFIX}/ ${GIMP_DISTRIB} --output-dll-list done-dll.list
   cp -fr ${MSYS_PREFIX}/lib/girepository-*/ ${GIMP_DISTRIB}/lib/