gitlab-ci: Make Homebrew builds scheduled only

Since the COMMIT_SHA, we default to build with MacPorts which is more native.

But it is wise to not be overly reliant on it, since it tends to be very flaky,
and Mac platform have very restrictive hardware with software always changing.

(cherry picked from commit 5abb249d1d)

.gitlab-ci.yml

@@ -16,6 +16,7 @@ spec:
       options:
         - GIMP_CI_MESON_CLANG #trigger the Debian Clang build (rare usefulness)
         - GIMP_CI_MESON_MSVC #trigger the Windows MSVC build (rare usefulness)
+        - GIMP_CI_MESON_APPLECLANG #trigger the Apple Clang build with Homebrew packages (rare usefulness)
         - GIMP_CI_RASTER_ICONS #trigger the Debian Clang build without vector icons (rare usefulness)
         - GIMP_CI_CPPCHECK #trigger cppcheck (static code analysis)
         - none
@@ -653,12 +654,17 @@ gimp-win-eol:
   rules:
     - if: '$CI_MERGE_REQUEST_LABELS =~ /.*Package:Macos Dmg.*/'
       interruptible: true
+      variables:
+        OPT_PREFIX: /opt/macports
     - if: '$GIMP_CI_MACOS != null || "$[[ inputs.distribution_pipeline ]]" =~ /.*GIMP_CI_MACOS.*/'
+      variables:
+        OPT_PREFIX: /opt/macports
+    - if: '($GIMP_CI_MESON_APPLECLANG != null || "$[[ inputs.test_pipeline ]]" =~ /.*GIMP_CI_MESON_APPLECLANG.*/) && $CI_JOB_NAME !~ /.*-part.*/ && $CI_JOB_NAME !~ /.*dist-mac-.*/'
+      variables:
+        OPT_PREFIX: /opt/homebrew
   tags:
     - macos
   variables:
-    #FIXME: Our runner #926 (by MacStadium) do not have cache nor timeout for MacPorts builds
-    OPT_PREFIX: /opt/macports
     PKGCONF_RELOCATABLE_OPTION: '-Dpkgconfig.relocatable=true'
     DMG_OPTION: '-Ddmg=true'
   before_script:
@@ -701,7 +707,10 @@ deps-macos-inhouse-part2:
 
 deps-macos-inhouse:
   extends: .macos-inhouse
-  needs: ["deps-macos-inhouse-part2"]
+  needs:
+    - job: deps-macos-inhouse-part2
+      #to allow running the Homebrew builds which use precompiled binaries
+      optional: true
   stage: dependencies
   script:
     # Do not call sh since it is old bash on macOS, not the default zsh