ci: use Cysharp/Actions checkout instead of 3rd party directly

Author: guitarrapc

.github/workflows/build-debug.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 10
     steps:
-      - uses: actions/checkout@v4
+      - uses: Cysharp/Actions/.github/actions/checkout@main
       - uses: Cysharp/Actions/.github/actions/setup-dotnet@main
       - run: dotnet build -c Debug
       - run: dotnet test -c Debug --no-build

.github/workflows/build-release.yml

@@ -17,13 +17,13 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 10
     steps:
-      - uses: actions/checkout@v4
+      - uses: Cysharp/Actions/.github/actions/checkout@main
       - uses: Cysharp/Actions/.github/actions/setup-dotnet@main
       - run: dotnet build -c Release -p:Version=${{ inputs.tag }}
       - run: dotnet test -c Release --no-build
       - run: dotnet pack -c Release --no-build -p:Version=${{ inputs.tag }} -o ./publish
       # Store artifacts.
-      - uses: actions/upload-artifact@v2
+      - uses: Cysharp/Actions/.github/actions/upload-artifact@main
         with:
           name: nuget
           path: ./publish/