cmd/app: centralise workdir

This makes the directory structure significantly more manageable.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/app/main.go

@@ -93,8 +93,8 @@ func main() {
 							{FilesystemConfig: &hst.FSOverlay{
 								Target: fhs.AbsRoot,
 								Lower:  []*check.Absolute{initial},
-								Upper:  template.Append(args[0], "upper"),
-								Work:   template.Append(args[0], "work"),
+								Upper:  template.Append(args[0]),
+								Work:   base.Append("work", args[0]),
 							}},
 							{FilesystemConfig: &hst.FSEphemeral{
 								Target: fhs.AbsTmp,