Workflows: Performance: Explicitly require Bash shell

The `run` block for the "Compare performance with current WordPress Core
and previous Gutenberg versions" job relies on a series of Bashisms, so
we should make that an explicit requirement.

.github/workflows/performance.yml

@@ -47,6 +47,7 @@ jobs:
               if: github.event_name == 'release'
               env:
                   PLUGIN_VERSION: ${{ github.event.release.name }}
+              shell: bash
               run: |
                   IFS=. read -ra PLUGIN_VERSION_ARRAY <<< "$PLUGIN_VERSION"
                   CURRENT_RELEASE_BRANCH="release/${PLUGIN_VERSION_ARRAY[0]}.${PLUGIN_VERSION_ARRAY[1]}"