gitlab-ci: Fix Windows artifact collection

Variable interpolation in gitlab-ci.yml apparently doesn't work. Sigh.

.gitlab-ci.yml

@@ -877,7 +877,9 @@ nightly-i386-windows-hadrian:
     reports:
       junit: junit.xml
     paths:
-      - $BIN_DIST_PREP_TAR_COMP
+      # N.B. variable interpolation apparently doesn't work on Windows so
+      # this can't be $BIN_DIST_TAR_COMP
+      - "ghc-x86_64-mingw32.tar.xz"
       - junit.xml
 
 validate-x86_64-windows: