Use github.ref_name as fallback for github.head_ref in e2e workflow

`github.head_ref` is only set on `pull_request` events; `workflow_dispatch` leaves it empty, whcih caused the `composer require` step to request an invalid "dev-" branch alias and fail.

Author: spaze

.github/workflows/e2e.yml

@@ -47,8 +47,8 @@ jobs:
       env:
         GH_TOKEN: ${{ github.token }}
 
-    - name: Update the extension to "dev-${{ github.head_ref }} as ${{ steps.latest.outputs.version }}"
-      run: composer require ${{ matrix.composer-params }} --dev "spaze/phpstan-disallowed-calls:dev-${{ github.head_ref }} as ${{ steps.latest.outputs.version }}" --with-all-dependencies
+    - name: Update the extension to "dev-${{ github.head_ref || github.ref_name }} as ${{ steps.latest.outputs.version }}"
+      run: composer require ${{ matrix.composer-params }} --dev "spaze/phpstan-disallowed-calls:dev-${{ github.head_ref || github.ref_name }} as ${{ steps.latest.outputs.version }}" --with-all-dependencies
 
     - name: Run tests
       run: ${{ matrix.run }}