If java JDK not installed, like it isn't in Github Codespaces, then rules_jvm_external fails

This fixes it: 

```
common --enable_platform_specific_config
# https://github.com/bazelbuild/rules_jvm_external/issues/445
build:linux --repo_env=JAVA_HOME=../bazel_tools/jdk
```

though that may super hacky and break in other ways.