Turn CI_ONLY_WHEN_CHANNEL from an environment variable to a job attribute

src/ci/github-actions/calculate-job-matrix.py

@@ -95,7 +95,7 @@ def skip_jobs(jobs: List[Dict[str, Any]], channel: str) -> List[Job]:
     """
     Skip CI jobs that are not supposed to be executed on the given `channel`.
     """
-    return [j for j in jobs if j.get("CI_ONLY_WHEN_CHANNEL", channel) == channel]
+    return [j for j in jobs if j.get("only_on_channel", channel) == channel]
 
 
 def get_github_ctx() -> GitHubCtx:

src/ci/github-actions/jobs.yml

@@ -200,24 +200,23 @@ auto:
   # channel name on the output), and this builder prevents landing
   # changes that would result in broken builds after a promotion.
   - image: x86_64-gnu-stable
+    # Only run this job on the nightly channel. Running this on beta
+    # could cause failures when `dev: 1` in `stage0.txt`, and running
+    # this on stable is useless.
+    only_on_channel: nightly
     env:
       IMAGE: x86_64-gnu
       RUST_CI_OVERRIDE_RELEASE_CHANNEL: stable
-      # Only run this job on the nightly channel. Running this on beta
-      # could cause failures when `dev: 1` in `stage0.txt`, and running
-      # this on stable is useless.
-      CI_ONLY_WHEN_CHANNEL: nightly
     <<: *job-linux-4c
 
   - image: x86_64-gnu-aux
     <<: *job-linux-4c
 
   - image: x86_64-gnu-integration
-    env:
-      # Only run this job on the nightly channel. Fuchsia requires
-      # nightly features to compile, and this job would fail if
-      # executed on beta and stable.
-      CI_ONLY_WHEN_CHANNEL: nightly
+    # Only run this job on the nightly channel. Fuchsia requires
+    # nightly features to compile, and this job would fail if
+    # executed on beta and stable.
+    only_on_channel: nightly
     <<: *job-linux-8c
 
   - image: x86_64-gnu-debug