ci: share one build cache scope across merge groups and pass the base ref down

Each merge group has a unique ref name, so keying the buildx cache on it wrote a
scope nothing reads again. The upgrade-from script derived the base branch on its
own from the same empty variable, so it now receives the one the action resolved.

Signed-off-by: Nicholas Bucher <behappy54321@gmail.com>

Author: Charlesthebird

.github/actions/upgrade-test-setup/action.yaml

@@ -31,11 +31,10 @@ runs:
       id: resolve
       shell: bash
       env:
-        MERGE_GROUP_BASE_REF: ${{ github.event.merge_group.base_ref }}
+        # A merge group has no base_ref; its ref name is gh-readonly-queue/<base>/pr-N-<sha>.
+        BASE_REF: ${{ github.base_ref || github.event.merge_group.base_ref || github.ref_name }}
       run: |
-        # A merge group has no GITHUB_BASE_REF; its ref name is gh-readonly-queue/<base>/pr-N-<sha>.
-        base_ref="${GITHUB_BASE_REF:-${MERGE_GROUP_BASE_REF#refs/heads/}}"
-        base_ref="${base_ref:-${GITHUB_REF_NAME:-}}"
+        base_ref="${BASE_REF#refs/heads/}"
         is_release_line=false
         if [[ "$base_ref" =~ ^release/v[0-9]+\.[0-9]+\.x$ ]]; then
           is_release_line=true
@@ -45,7 +44,7 @@ runs:
           # prev-stable = the previous minor line's latest published version.
           # Before that line has a final release, use its latest prerelease so
           # main keeps upgrade coverage after the release branch is cut.
-          V="$(./scripts/prev-stable-version.sh)"
+          V="$(CURRENT_REF="$base_ref" ./scripts/prev-stable-version.sh)"
           if [ -z "$V" ]; then
             echo "no published version on the previous release line; skipping prev-stable leg."
             echo "skip=true" >> "$GITHUB_OUTPUT"

.github/workflows/ci.yaml

@@ -19,7 +19,8 @@ concurrency:
 env:
   # Cache key components for better organization
   CACHE_KEY_PREFIX: kagent-v2
-  BRANCH_CACHE_KEY: ${{ github.head_ref || github.ref_name }}
+  # Merge groups share one cache scope; their ref names are unique and would each write a throwaway one.
+  BRANCH_CACHE_KEY: ${{ github.head_ref || (github.event.merge_group.base_ref && 'merge-queue') || github.ref_name }}
   # Consistent builder configuration
   BUILDX_BUILDER_NAME: kagent-builder-v0.23.0
   BUILDX_VERSION: v0.23.0

.github/workflows/migration-immutability.yaml

@@ -17,6 +17,7 @@ jobs:
 
       - name: Fail if any existing migration file was modified
         env:
+          # A merge group has no base_ref; it only exposes the base branch as refs/heads/<branch>.
           BASE_REF: ${{ github.base_ref || github.event.merge_group.base_ref }}
         run: |
           BASE=$(git merge-base HEAD "origin/${BASE_REF#refs/heads/}")