Cleanup RBE logs directory

RBE logs directory is currently messy:
1. We use RBE_output_dir variable to specify where rbe_metrics.txt /
   rbe_metrics.pb file should go to.
2. We use proxy_log_dir to specify where *.rpl / *.rpi (detailed per
   action info log file) should go to.
3. We use RBE_log_dir to specify where reproxy.* / bootstrap.* log files
   should go to.

Ideally, all RBE related logs should go to one single directory. In this
CL, I'm creating a temporary log directory under out/soong/.temp/rbe/
where all RBE related log files per build would go to. The log dir
prefix is also being set to the same prefix as the socket address file.

Test: Ran a sample build with `m libc` and ensured that logs are getting
cleared across rebuilds and that `rbe_metrics.pb` file is properly
generated and being copied to the right location (i.e., from
out/soong/.temp/rbe/<rand> to out/ dir)

Bug: b/233382420
Change-Id: I46bd38d50419cb9e54e8202d24222979e47ff5ca

ui/build/build.go

@@ -266,6 +266,7 @@ func Build(ctx Context, config Config) {
 	}
 
 	if config.StartRBE() {
+		cleanupRBELogsDir(ctx, config)
 		startRBE(ctx, config)
 		defer DumpRBEMetrics(ctx, config, filepath.Join(config.LogsDir(), "rbe_metrics.pb"))
 	}