Create missing ~/.stack for alpine unit-tests.yml

The GitHub Action unit-tests.yml is failing with:

~~~
+ [[ alpine == \a\l\p\i\n\e ]]
+ touch /home/runner/.stack/config.yaml
touch: cannot touch '/home/runner/.stack/config.yaml': No such file or directory
Error: Process completed with exit code 1.
~~~

This attempts to fix that by adding: `mkdir -p ~/.stack`

Author: mpilgrem

.github/workflows/unit-tests.yml

@@ -75,6 +75,7 @@ jobs:
 
         if [[ "${{ matrix.extra-suffix }}" == "alpine" ]]
         then
+            mkdir -p ~/.stack
             touch ~/.stack/config.yaml
             cat > ~/.stack/config.yaml <<EOF
             extra-include-dirs: