update build-flutter template

templates/build-flutter.yml

@@ -2,6 +2,8 @@
 
 # Starting from the following template and grep for "flutter" in the metadata
 # directory to see other examples.
+# Only choose one method between flutter submodule and srclib. If submodule method
+# is chosen then please add flutter as a submodule in the source repo.
 
 Builds:
   - versionName: '1.0'
@@ -27,10 +29,9 @@ Builds:
     prebuild:
       # Extract the flutter version and then checkout it in the flutter srclib
       # Only needed when flutter srclib is used
-      - version=$(sed -n -E "s/.*flutter-version:\ '(.*)'/\1/p" .github/workflows/release.yml)
-      - pushd $$flutter$$
-      - git checkout -f $version
-      - popd
+      - flutterVersion=$(sed -n -E "s/.*flutter-version:\ '(.*)'/\1/p" .github/workflows/release.yml)
+      - '[[ $flutterVersion ]] || exit 1'
+      - git -C $$flutter$$ checkout -f $flutterVersion
       # Change pub cache location so that the dart packages are scanned by the scanner
       - export PUB_CACHE=$(pwd)/.pub-cache
       - .flutter/bin/flutter config --no-analytics