Consider paths passed to x.py to be root-relative.

We'd previously assumed that these paths would be relative to the src
dir, and that for example our various CI scripts would, when calling
x.py, use `../x.py build ../src/tools/...` but this isn't the case --
they use `../x.py` without using the relevant source-relative path.

We eventually may want to make this (actually somewhat logical) change,
but this is not that time.

src/bootstrap/builder.rs

@@ -385,6 +385,10 @@ impl<'a> Builder<'a> {
             Subcommand::Clean { .. } => panic!(),
         };
 
+        if paths[0] == Path::new("nonexistent/path/to/trigger/cargo/metadata") {
+            return;
+        }
+
         let builder = Builder {
             build,
             top_stage: build.config.stage.unwrap_or(2),