ci: Make per-build dependencies optional

Sometimes not all of the jobs execute. For instance, Windows build jobs
will not trigger on AMD-only MRs. Use the 'optional' keyword to ignore
jobs which don't exist in our pipeline.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Fixes: 310e3bb026 ("ci: do not start build-only jobs until the critical build-for-tests jobs are done")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30926>

.gitlab-ci/build/gitlab-ci.yml

@@ -66,13 +66,20 @@
 # Make sure this list stays the same as all the jobs with
 # `stage: build-for-tests`
 .build-for-tests-jobs:
-  - debian-testing
-  - debian-testing-asan
-  - debian-arm32
-  - debian-arm32-asan
-  - debian-arm64
-  - debian-arm64-asan
-  - windows-msvc
+  - job: debian-testing
+    optional: true
+  - job: debian-testing-asan
+    optional: true
+  - job: debian-arm32
+    optional: true
+  - job: debian-arm32-asan
+    optional: true
+  - job: debian-arm64
+    optional: true
+  - job: debian-arm64-asan
+    optional: true
+  - job: windows-msvc
+    optional: true
 
 
 debian-testing: