sdk.properties is often (always?) stale.

I'm not sure when sdk.properties is rebuilt, but it is not often (perhaps it is only regenerated on a clean build). `./gradlew :sdks:java:core:processResources` never does anything, it always reports being up to date.

One consequence of this is that when the SDK version property in Gradle changes, sdk.properties does not change. We rely on sdk.properties to get the default Java SDK container version tag, among other things, so this causes the default container environment to become incorrect.

For example, this caused cross language tests to fail on the release branch because they looked for the wrong Java SDK container. https://ci-beam.apache.org/job/beam_PostCommit_Python38_PR/179/