Do not delete build cache when building release candidates (#1619)

We would like to re-use the build cache when building RCs for different
environments. There's not much practical use in doing a "clean" for
every build when Gradle should be able to figure out which artifacts
need to be rebuilt. It also does not make sense to build each
environment in a separate step, which also introduces redunency because
not all artifacts are cached across steps. The build cache is enabled by
default.

Lastly, the cache needs to be inside the /workspace folder, which is the
default persisted storage location.

TESTED=tried to build the RCs on alpha and saved about 10 min.

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gradle.properties

@@ -8,6 +8,7 @@
 #
 # DO NOT EDIT THIS FILE BY HAND
 org.gradle.jvmargs=-Xmx1024m
+org.gradle.caching=true
 mavenUrl=
 pluginsUrl=
 uploaderDestination=