Normalizing output doesn't work properly in Windows

The following error shows that normalizing is not working as expected:
```
-Path '/warn-early-overwrite' already exists. Use --overwrite-policy=always to overwrite.
+Path '' already exists. Use --overwrite-policy=always to overwrite.
# cabal v2-install
Wrote tarball sdist to /dirty-install.dist/work/./dist/sdist/WarnEarlyOverwrite-0.1.0.0.tar.gz
Resolving dependencies...
@@ -16,5 +16,5 @@
Preprocessing executable 'warn-early-overwrite' for WarnEarlyOverwrite-0.1.0.0...
Building executable 'warn-early-overwrite' for WarnEarlyOverwrite-0.1.0.0...
Installing executable warn-early-overwrite in
-Warning: The directory /ghc-/incoming/new-/ghc-/-/bin is not in the system search path.
-Symlinking 'warn-early-overwrite' to '/warn-early-overwrite'
+Warning: The directory /incoming/new-109496/msys64/tmp/cabal-test-store-93596/ghc-/WarnEarlyOver_-0.1.0.0-05647f1fc6a6fbf23e68db895ff877577f98f68a/bin is not in the system search path.
+Symlinking 'warn-early-overwrite' to ''
```

The affected tests are:

- PackageTests\WarnEarlyOverwrite\dirty-install.test.hs
- PackageTests\WarnEarlyOverwrite\clean-install-by-symlink.test.hs
- PackageTests\WarnEarlyOverwrite\clean-install-by-copy.test.hs