pw_build: Fix issue with temp dirs on Mac

Change-Id: Id872ffd09d26bd2b62e866d290ab785c2b45dfab
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/16705
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Rob Mohr <mohrr@google.com>
diff --git a/pw_build/py/pw_build/python_runner.py b/pw_build/py/pw_build/python_runner.py
index e627bb0..0de9b1a 100755
--- a/pw_build/py/pw_build/python_runner.py
+++ b/pw_build/py/pw_build/python_runner.py
@@ -124,7 +124,7 @@
 
         # Resolve the directory to an absolute path
         set_attr('dir', paths.resolve(directory))
-        set_attr('relative_dir', self.dir.relative_to(paths.root))
+        set_attr('relative_dir', self.dir.relative_to(paths.root.resolve()))
 
         set_attr(
             'out_dir',