Android OSS: fixed unbound variable error for codegen build script

Summary:
If $FBSOURCE_ENV isn't set, this failed with `/root/react-native/packages/react-native-codegen/android/../scripts/oss/build.sh: line 20: FBSOURCE_ENV: unbound variable`.

This commit fixes https://app.circleci.com/pipelines/github/facebook/react-native/7080/workflows/5cf18a1f-e6d2-4648-8217-d42e9a61fef1/jobs/176451

Changelog: [Internal]

Reviewed By: yungsters

Differential Revision: D24912950

fbshipit-source-id: 113e3dd7f78c75fc3adea0b21c9e38910be8c065

packages/react-native-codegen/scripts/oss/build.sh

@@ -17,7 +17,7 @@ rm -rf "${CODEGEN_DIR:?}/lib" "${CODEGEN_DIR:?}/node_modules"
 
 YARN_BINARY="${YARN_BINARY:-$(command -v yarn)}"
 
-if [[ "$FBSOURCE_ENV" -eq "1" ]]; then
+if [[ ${FBSOURCE_ENV:-0} -eq 1 ]]; then
   # Custom FB-specific setup
   pushd "$CODEGEN_DIR" >/dev/null