Substitute github context variables in CI email template

.github/workflows/ci_failure_email.md.tmpl

@@ -0,0 +1,3 @@
+CI workflow ${WORKFLOW} failed!
+
+The failed job can be found in [here](${SERVER_URL}/${REPOSITORY}/actions/runs/${RUN_ID}).

.github/workflows/push.yaml

@@ -14,6 +14,15 @@ jobs:
         uses: shivammathur/setup-php@v2
         with:
           php-version: ${{ matrix.php_version }}
+      - uses: nowactions/envsubst@v1
+        with:
+          input: ${{ github.workspace }}/.github/workflows/ci_failure_email.md.tmpl
+          output: ${{ github.workspace }}/.github/workflows/ci_failure_email.md
+        env:
+          WORKFLOW: ${{ github.workflow }}
+          SERVER_URL: ${{ github.server_url }}
+          REPOSITORY: ${{ github.repository }}
+          RUN_ID: ${{ github.run_id }}
       - name: Validate composer.json and composer.lock
         run: composer validate --strict
       - name: Cache Composer packages
@@ -38,4 +47,4 @@ jobs:
           from: ${{ github.repository }} CI
           subject: CI job failed for ${{ github.repository }}
           convert_markdown: true
-          html_body: file:///home/runner/work/Diff/Diff/.github/workflows/ci_failure_email.md
+          html_body: file://${{ github.workspace }}/.github/workflows/ci_failure_email.md