Use property to control build cache push instead of CI env variable s… (#998)

* Use property to control build cache push instead of CI env variable since many builds have that set.

* Check env instead.

settings.gradle

@@ -23,7 +23,7 @@ buildCache {
   remote(com.github.burrunan.s3cache.AwsS3BuildCache) {
     region = 'us-west-2'
     bucket = 'opentelemetry-java-instrumentation-gradle-test1'
-    push = isCI
+    push = System.getenv('S3_BUILD_CACHE_ACCESS_KEY_ID') != null
   }
 }