ci: cache prepackaged plugins in mmctl tests

Cache the prepackaged_plugins directory between mmctl test runs using
actions/cache. The cache key is derived from a SHA-256 hash of the
PLUGIN_PACKAGES lines in server/Makefile, so the cache automatically
invalidates whenever any plugin version is bumped — no manual key
updates needed.

Uses actions/cache output (cache-hit) to skip the download step
entirely on cache hits.

Saves ~1-2 minutes per mmctl test run by avoiding repeated plugin
downloads.

Co-authored-by: Claude <claude@anthropic.com>

.github/workflows/mmctl-test-template.yml

@@ -54,7 +54,23 @@ jobs:
         run: |
           echo "${{ inputs.name }}" > server/test-name
           echo "${{ github.event.pull_request.number }}" > server/pr-number
+      - name: Compute plugin cache key
+        id: plugin-cache-key
+        run: |
+          # Extract PLUGIN_PACKAGES lines from Makefile to derive a stable cache key.
+          # Cache auto-invalidates whenever any plugin version changes in the Makefile.
+          PLUGIN_HASH=$(grep '^PLUGIN_PACKAGES' server/Makefile | sha256sum | cut -d' ' -f1)
+          echo "hash=$PLUGIN_HASH" >> "$GITHUB_OUTPUT"
+
+      - name: Cache prepackaged plugins
+        id: plugin-cache
+        uses: actions/cache@5a3ec84eff668545956fd18022155c47e93e2684 # v4.2.3
+        with:
+          path: server/prepackaged_plugins
+          key: prepackaged-plugins-${{ steps.plugin-cache-key.outputs.hash }}
+
       - name: Setup needed prepackaged plugins
+        if: steps.plugin-cache.outputs.cache-hit != 'true'
         run: |
           cd server
           make prepackaged-plugins PLUGIN_PACKAGES=mattermost-plugin-jira-v3.2.5