Fix MYPYPATH declaration

Per chance, it worked for the maxos and linux jobs, but not for the
windows one.

Don't know how `CI_PROJECT_DIR` snuck in, seems like that's the name
GitLab would use..

Author: lagru

.github/workflows/ci.yml

@@ -15,7 +15,12 @@ env:
   # Many color libraries just need this to be set to any value, but at least
   # one distinguishes color depth, where "3" -> "256-bit color".
   FORCE_COLOR: 3
-  MYPYPATH: ${CI_PROJECT_DIR}/stubs
+  MYPYPATH: ${{ github.workspace }}/stubs
+
+defaults:
+  run:
+    # Make sure that bash specific stuff works on Windows
+    shell: bash
 
 jobs:
   lint: