Fix Android template E2E for PRs targeting stable branches (#57650)

On pull_request events github.ref_name is "<pr-number>/merge", which never
matches *-stable*, so the template E2E always cloned the template's main
branch. Template main now ships a Gradle 9.4.1 wrapper (bundling Kotlin
2.3.0), whose metadata the Kotlin 2.1.0 compiler used by
@react-native/gradle-plugin on 0.86 cannot read, breaking
:gradle-plugin:settings-plugin:compileKotlin on every PR to 0.86-stable.

Use github.base_ref (set on PR events) so PRs targeting stable branches
check out the matching template branch, falling back to ref_name for
branch pushes.

Author: gabrieldonadel

.github/workflows/e2e-android-templateapp.yml

@@ -56,9 +56,11 @@ jobs:
           MAVEN_LOCAL=/tmp/react-native-tmp/maven-local
           echo "Maven local path is $MAVEN_LOCAL"
 
-          # For stable branches, we want to use the stable branch of the template
-          # In all the other cases, we want to use "main"
-          BRANCH=${{ github.ref_name }}
+          # For stable branches, we want to use the stable branch of the template.
+          # In all the other cases, we want to use "main".
+          # On pull_request events, ref_name is "<pr-number>/merge", so use the
+          # PR base branch instead to keep targeting the matching template branch.
+          BRANCH=${{ github.base_ref || github.ref_name }}
           if ! [[ $BRANCH == *-stable* ]]; then
             BRANCH=main
           fi