ci: fix develop PR build signing (validateSigningDevelop) (#4)

* ci: wire develop_key.jks into PR build (fix validateSigningDevelop)

The PR build runs assembleDevelop (signingConfigs.dev = develop_key.jks) but the
reusable workflow only decoded github/market keystores, so validateSigningDevelop
always failed. Add a develop-keystore decode step (reusing the existing
BASE64_DEV_KEYSTORE_FILE secret + CI_KEYSTORE_* passwords) and pass
keystore-file-name: develop_key.jks from pull_request.yml.

[temp] pin reusable workflow to the branch to validate before merge; reverted to
@main in the next commit.

* ci: revert reusable-workflow pin back to @main

Fix validated green on PR #4 (test/Build app and test passed, develop signing
works). Restore @main pin for the final merged state.

.github/workflows/pull_request.yml

@@ -10,5 +10,6 @@ jobs:
     with:
       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
     secrets: inherit