Revert "Use directory symlink instead of junction for Dev Drive"

This reverts commit fce8d2d.

Author: pietern

.github/actions/setup-build-environment/action.yml

@@ -47,12 +47,7 @@ runs:
         }
 
         New-Item -ItemType Directory -Path Z:\fast | Out-Null
-        # Use a directory symbolic link (not a junction). Go's
-        # `filepath.EvalSymlinks` handles dir symlinks correctly but returns
-        # "cannot find the path specified" on junctions rooted at a newly
-        # mounted VHDX (hits `bundle.TestRootLookup` in the unit test suite).
-        # Requires Developer Mode, which is enabled on GH Windows runners.
-        cmd /c "mklink /D C:\a\_fast Z:\fast" | Out-Null
+        New-Item -ItemType Junction -Path C:\a\_fast -Target Z:\fast | Out-Null
         New-Item -ItemType Directory -Path `
           C:\a\_fast\tmp, C:\a\_fast\go-build, C:\a\_fast\go-mod, C:\a\_fast\go-tmp | Out-Null