CI: Set up cache for Cygwin pip

Copied from the MIT-licensed:
https://github.com/actions/cache/blob/main/examples.md#using-pip-to-get-cache-location

.github/workflows/test-cygwin.yml

@@ -44,6 +44,19 @@ jobs:
         with:
           dirs: 'C:\cygwin\bin;C:\cygwin\lib\lapack'
           
+      - name: Get pip cache dir
+        id: pip-cache
+        run: |
+          bash.exe -c 'echo "::set-output name=dir::$(pip cache dir)"'
+          
+      - name: pip cache
+        uses: actions/cache@v3
+        with:
+          path: ${{ steps.pip-cache.outputs.dir }}
+          key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements.txt') }}
+          restore-keys: |
+            ${{ runner.os }}-pip-
+
       - name: Build system information
         run: |
           bash.exe -c "python3 .github/workflows/system-info.py"