Fix Android template E2E for PRs targeting stable branches

[gabrieldonadel]

Summary:

test_e2e_android_templateapp currently fails on PRs targeting 0.86-stable (both debug and release flavors), e.g. https://github.com/react/react-native/actions/runs/30006334544/job/89219502470?pr=57646.

The workflow picks which branch of react-native-community/template to generate the test project from:

BRANCH=${{ github.ref_name }}
if ! [[ $BRANCH == *-stable* ]]; then
  BRANCH=main
fi

On pull_request events github.ref_name is <pr-number>/merge, which never matches *-stable*, so PR runs always clone the template's main branch. That guard only works for direct pushes to the stable branch.

This recently started failing because template main bumped its Gradle wrapper to 9.4.1, which bundles the Kotlin 2.3.0 stdlib. The generated project compiles @react-native/gradle-plugin from source with Kotlin 2.1.0 on 0.86, and that compiler can only read Kotlin metadata up to 2.2.0, so :gradle-plugin:settings-plugin:compileKotlin fails with:

Module was compiled with an incompatible version of Kotlin.
The binary version of its metadata is 2.3.0, expected version is 2.1.0.

The fix is to use github.base_ref (the PR target branch, e.g. 0.86-stable) when it is set, falling back to github.ref_name for branch pushes:

BRANCH=${{ github.base_ref || github.ref_name }}

With this, PRs targeting 0.86-stable check out the template's 0.86-stable branch (Gradle 9.3.1 / Kotlin 2.2 metadata, readable by the 2.1.0 compiler), matching the intent of the original comment in the workflow.

Note: e2e-ios-templateapp.yml has the same pattern (currently not failing since the Kotlin issue is Android-only), and main carries the same logic in both files, so future stable branches will inherit this. Happy to follow up there if desired.

Changelog:

[INTERNAL] [FIXED] - Use the PR base branch to pick the template branch in the Android template E2E workflow

Test Plan:

• Verified scripts/e2e/init-project-e2e.js does a plain git checkout of the value passed as --currentBranch in the cloned template repo, and that the 0.86-stable branch exists in react-native-community/template (with a Gradle 9.3.1 wrapper, vs 9.4.1 on template main).
• CI on this PR is the real test: test_e2e_android_templateapp should now generate the project from the template's 0.86-stable branch and pass, while all other jobs remain unaffected.