artifact: implement findHomeDir helper as specified

This makes a couple small changes to the findHomeDir helper function
to properly match the described behavior. It will attempt to use
os.UserHomeDir first, and if no path can be discovered, it will return
/dev/null.

Author: chrisroberts

client/allocrunner/taskrunner/getter/util_linux.go

@@ -28,6 +28,10 @@ var initialDirs = map[string]string{
 // os.UserHomeDir returns an error, we return /root if the current process is being
 // run by root, or /dev/null otherwise.
 func findHomeDir() string {
+	if home, err := os.UserHomeDir(); err == nil {
+		return home
+	}
+
 	// When running as a systemd unit the process may not have the $HOME
 	// environment variable set, and os.UserHomeDir will return an error.
 
@@ -42,7 +46,7 @@ func findHomeDir() string {
 	}
 
 	// nothing safe to do
-	return "/nonexistent"
+	return "/dev/null"
 }
 
 // defaultEnvironment is the default minimal environment variables for Linux.