packaging: make the path to Win32 requirements absolute when building WiX

Otherwise this broke automation when not launched from `contrib/packaging`.

Differential Revision: https://phab.mercurial-scm.org/D8068

contrib/packaging/hgpackaging/wix.py

@@ -343,7 +343,9 b' def build_installer('
     dist_dir = source_dir / 'dist'
     wix_dir = source_dir / 'contrib' / 'packaging' / 'wix'
 
-    requirements_txt = 'requirements_win32.txt'
+    requirements_txt = (
+        source_dir / 'contrib' / 'packaging' / 'requirements_win32.txt'
+    )
 
     build_py2exe(
         source_dir,