Delete jmod workDir

Files in this directory aren't exposed to the rest of the build, so
deleting them saves some space. The `jmod create` command was also
producing non-deterministic files, which are not relevant because
they're not used anywhere else, but it makes it easier to find
non-determinism that matters if we delete them.

Test: Presubmits
Change-Id: I5d57826b438368f24a7a2a7ab1ccd6c16f240124

java/system_modules.go

@@ -64,6 +64,7 @@ var (
 			// useful on Android, and (b) it causes errors with later versions of jlink
 			// when the jdk.internal.module is absent from java.base (as it is here).
 			`  --disable-plugin system-modules && ` +
+			`rm -rf ${workDir} && ` +
 			`cp ${config.JrtFsJar} ${outDir}/lib/`,
 		CommandDeps: []string{
 			"${moduleInfoJavaPath}",