add the x86_64-gnu-stable job to test with stable channel

During the 1.52 release process we had to deal with some commits that
passed the test suite on the nightly branch but failed on the beta or
stable branch. In that case it was due to some UI tests including the
channel name in the output, but other changes might also be dependent on
the channel.

This commit adds a new CI job that runs the Linux x86_64 test suite with
the stable branch, ensuring nightly changes also work as stable.

.github/workflows/ci.yml

@@ -259,6 +259,11 @@ jobs:
           - name: x86_64-gnu
             os: ubuntu-latest-xl
             env: {}
+          - name: x86_64-gnu-stable
+            env:
+              IMAGE: x86_64-gnu
+              RUST_CI_OVERRIDE_RELEASE_CHANNEL: stable
+            os: ubuntu-latest-xl
           - name: x86_64-gnu-aux
             os: ubuntu-latest-xl
             env: {}

src/ci/docker/run.sh

@@ -235,6 +235,7 @@ docker \
   --env TOOLSTATE_REPO_ACCESS_TOKEN \
   --env TOOLSTATE_REPO \
   --env TOOLSTATE_PUBLISH \
+  --env RUST_CI_OVERRIDE_RELEASE_CHANNEL \
   --env CI_JOB_NAME="${CI_JOB_NAME-$IMAGE}" \
   --init \
   --rm \

src/ci/github-actions/ci.yml

@@ -407,6 +407,17 @@ jobs:
           - name: x86_64-gnu
             <<: *job-linux-xl
 
+          # This job ensures commits landing on nightly still pass the full
+          # test suite on the stable channel. There are some UI tests that
+          # depend on the channel being built (for example if they include the
+          # channel name on the output), and this builder prevents landing
+          # changes that would result in broken builds after a promotion.
+          - name: x86_64-gnu-stable
+            env:
+              IMAGE: x86_64-gnu
+              RUST_CI_OVERRIDE_RELEASE_CHANNEL: stable
+            <<: *job-linux-xl
+
           - name: x86_64-gnu-aux
             <<: *job-linux-xl
 

src/ci/run.sh

@@ -65,7 +65,11 @@ fi
 # Always set the release channel for bootstrap; this is normally not important (i.e., only dist
 # builds would seem to matter) but in practice bootstrap wants to know whether we're targeting
 # master, beta, or stable with a build to determine whether to run some checks (notably toolstate).
-export RUST_RELEASE_CHANNEL="$(cat "${ci_dir}/channel")"
+if [[ -z "${RUST_CI_OVERRIDE_RELEASE_CHANNEL+x}" ]]; then
+    export RUST_RELEASE_CHANNEL="$(cat "${ci_dir}/channel")"
+else
+    export RUST_RELEASE_CHANNEL="${RUST_CI_OVERRIDE_RELEASE_CHANNEL}"
+fi
 RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --release-channel=$RUST_RELEASE_CHANNEL"
 
 if [ "$DEPLOY$DEPLOY_ALT" = "1" ]; then