Fix problem with pip 21.3 that prevents installation from source (#2344)

* Add test-fix branch to ga

* Pin pip to avoid pypa/pip#10573

Author: Klaus Zimmermann

.github/workflows/action-install-from-source.yml

@@ -5,6 +5,7 @@ on:
   push:
     branches:
     - main
+    - test-fix
   schedule:
     - cron: '0 0 * * *'
 

environment.yml

@@ -4,6 +4,7 @@ channels:
   - conda-forge
 
 dependencies:
+  - pip!=21.3
   # Python packages that cannot be installed from PyPI:
   - cartopy>=0.18
   - compilers