ci: add workflow_dispatch escape hatch to pull_request.yml (#8)

pull_request-triggered runs of this workflow have stopped firing
entirely for at least one recent PR (no check-suite created at all
for the GitHub Actions app, while two other installed apps sit stuck
in "queued") - root cause is outside what's fixable via the repo's
own Actions config/API access. This adds a manual workflow_dispatch
path (branch input) so a build+test run can still be triggered
directly against any branch while that's being sorted out.

.github/workflows/pull_request.yml

@@ -2,13 +2,20 @@ name: Pull request
 
 on:
   pull_request:
+  workflow_dispatch:
+    inputs:
+      branch:
+        description: 'Branch to build and test (manual escape hatch for when pull_request-triggered CI does not fire)'
+        required: true
+        default: 'main'
+        type: string
 
 
 jobs:
   test:
     uses: pezkuwichain/pezkuwi-wallet-android/.github/workflows/android_build.yml@main
     with:
-      branch: ${{github.head_ref}}
+      branch: ${{ github.event_name == 'workflow_dispatch' && inputs.branch || github.head_ref }}
       gradlew-command: assembleDevelop
       keystore-file-name: develop_key.jks # develop variant is signed with signingConfigs.dev (develop_key.jks)
       build-debug-tests: false # TODO: Enable this, when debug build will be fixed for tests