limit ndk jobs for RNTester on CI (#19755)

Summary:
Limit number of NDK jobs of ReactAndroid on CI using $BUILD_THREADS environment variable. Otherwise, it was spawning 32 jobs while building RNTester, which caused in OOM or unexpected failure.

CI: https://circleci.com/gh/dulmandakh/react-native/387

Android CI is green again 😍
Closes #19755

Differential Revision: D8466069

Pulled By: hramos

fbshipit-source-id: fd507d12c8e93f718e047719daa68aa25c37bfb2

.circleci/config.yml

@@ -569,7 +569,7 @@ jobs:
       - run:
           name: Build Android RNTester
           command: |
-            ./gradlew RNTester:android:app:assembleRelease
+            ./gradlew RNTester:android:app:assembleRelease -Pjobs=$BUILD_THREADS
 
       # Collect Results
       - run: *collect-android-test-results