Re-enable creation of AppVeyor artifacts to provide nightly builds

Fixes #2651.

They were disabled in 05116b8311 in 2019.
But since then in 2021 the retention [policy] has changed and artifacts
are now kept only for 1 month (for free accounts).

[policy]: https://www.appveyor.com/blog/2021/03/30/artifacts-retention-policy-update/

appveyor.yml

@@ -258,10 +258,10 @@ after_build:
 #      artifacts configuration    #
 #---------------------------------#
 
-#artifacts:
-#  - path: '**\*.exe'
-#  - path: filelist.txt
-# - path: $(RS_DEPLOY)
+artifacts:
+  - path: '**\*.exe'
+  - path: filelist.txt
+  - path: $(RS_DEPLOY)
 
 #
 #  # pushing a single file