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

@@ -24,7 +24,7 @@ var initialDirs = map[string]string{
 	"/usr/libexec":   "rx",
 }
 
-// findHomeDir returns the home directory as provided by os.UserHomeDir. In case
+// findHomeDir returns the home directory as provided by homedir.Dir(). In case
 // os.UserHomeDir returns an error, we return /root if the current process is being
 // run by root, or /dev/null otherwise.
 func findHomeDir() string {
@@ -42,7 +42,7 @@ func findHomeDir() string {
 	}
 
 	// nothing safe to do
-	return "/nonexistent"
+	return "/dev/null"
 }
 
 // findConfigDir returns the config directory as provided by os.UserConfigDir. In