build/linux: Re-enable ccache in Flatpak builds

The move to Clang in 85ed2847 didn't take into account caching,
Now, this is fixed or, better saying, reintroduced(?): e545116b

Also, flatpak-builder output was confined to the .log file to make
possible checking the end of the CI output, which was being cut.

.gitlab-ci.yml

@@ -350,14 +350,11 @@ gimp-flatpak-x64:
     APP_ID: "org.gimp.GIMP"
     BUNDLE: "gimp-git.flatpak"
   cache:
-    - key: "$CI_JOB_NAME"
+    - key: ${CI_JOB_NAME}-pkg
       paths:
-        - '.flatpak-builder/downloads'
-        - '.flatpak-builder/git'
-    - key: "$CI_JOB_NAME"
-      paths:
-        - '.flatpak-builder/cache'
-        - '.flatpak-builder/ccache'
+        - .flatpak-builder/downloads
+        - .flatpak-builder/git
+    - !reference [.default, cache]
   script:
     # Report the installed versions of the runtime
     - flatpak info org.gnome.Platform
@@ -383,8 +380,7 @@ gimp-flatpak-x64:
     # 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.)
-    - flatpak-builder --user --disable-rofiles-fuse flatpak_app --repo=repo ${BRANCH:+--default-branch=$BRANCH} ${MANIFEST_PATH}
-        2>&1 | tee flatpak-builder.log
+    - flatpak-builder --ccache --user --disable-rofiles-fuse flatpak_app --repo=repo ${BRANCH:+--default-branch=$BRANCH} ${MANIFEST_PATH} &>flatpak-builder.log
 
     # Generate a Flatpak bundle
     - flatpak build-bundle repo ${BUNDLE} --runtime-repo=${RUNTIME_REPO} ${APP_ID} ${BRANCH}

build/linux/flatpak/org.gimp.GIMP-nightly.json.in

@@ -12,8 +12,8 @@
         "append-path": "/usr/lib/sdk/llvm17/bin",
         "prepend-ld-library-path": "/usr/lib/sdk/llvm17/lib",
         "env": {
-            "CC": "clang",
-            "CXX": "clang++"
+            "CC": "ccache clang",
+            "CXX": "ccache clang++"
         }
     },
     "separate-locales": false,