gitlab-ci: Fix Windows bindist collection

Apparently variable interpolation in the `artifacts.paths` key of
`gitlab-ci.yml` doesn't work on Windows as it does on WIndows.

(cherry picked from commit 100cc756)

.gitlab-ci.yml

@@ -878,7 +878,7 @@ nightly-i386-windows-hadrian:
     reports:
       junit: junit.xml
     paths:
-      - $BIN_DIST_PREP_TAR_COMP
+      - "ghc-x86_64-mingw32.tar.xz"
       - junit.xml
 
 validate-x86_64-windows: