GHA: Fix cache directory on Windows (#97)

Previous settings worked only on linux and macos...

Author: dweindl

.github/workflows/ci_tests.yml

@@ -23,10 +23,15 @@ jobs:
       with:
         python-version: ${{ matrix.python-version }}
 
+    - name: Get pip cache directory
+      id: pip_cache_dir
+      run: |
+        echo "::set-output name=dir::$(pip cache dir)"
+
     - name: Cache
       uses: actions/cache@v1
       with:
-        path: ~/.cache/pip
+        path: ${{ steps.pip_cache_dir.outputs.dir }}
         key: ${{ runner.os }}-${{ matrix.python-version }}-${{ hashFiles('**/.ci_pip_reqs.txt') }}-${{ hashFiles('**/setup.py') }}
         restore-keys: |
           ${{ runner.os }}-${{ matrix.python-version }}-