Fix MYPYPATH declaration

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 +15,7 @@ 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"
 
 jobs:
   lint: