github/submodule-update: Reference env as a context thing.

.github/workflows/submodule-update.yml

@@ -62,7 +62,7 @@ jobs:
               owner: '${{ github.repository_owner }}',
               repo: '${{ github.repository }}'.split('/')[1].trim(),
               head: 'submodule-change/${{ github.run_id }}',
-              base: '${ process.env.PR_AGAINST_BRANCH }',
+              base: '${{ env.PR_AGAINST_BRANCH }}',
               title: `[Auto-generated] Submodule Updates ${process.env.GITHUB_RUN_ID}`,
               body: '${{ steps.check_for_changes.outputs.changes_text }}',
             });