Cache GitHub Actions (#565)

Improved `commit` workflow job times from an average of 8m to:

- 6m 30s uncached
- 5m 30s cached

Times were improved by:

- Adding caching
- Installing packages to the `D:\` drive, as described in <actions/setup-dotnet#260 (comment)>

.github/workflows/commit.yml

@@ -15,6 +15,11 @@ on:
       - ".github/pull_request_template.md"
       - "README.md"
 
+env:
+  # We install the packages to the D:\ drive to avoid the slow IO on the C:\ drive.
+  # Based on https://github.com/actions/setup-dotnet/issues/260#issuecomment-1790162905
+  NUGET_PACKAGES: D:\a\.nuget\packages
+
 jobs:
   commit:
     strategy:
@@ -26,7 +31,16 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
+
+      - name: Cache NuGet packages
+        uses: actions/cache@v3
+        with:
+          path: ${{ env.NUGET_PACKAGES }}
+          key: ${{ matrix.platform }}-nuget-${{ hashFiles('Directory.Packages.props') }}
+          restore-keys: |
+            ${{ matrix.platform }}-nuget-${{ hashFiles('Directory.Packages.props') }}
+            ${{ matrix.platform }}-nuget-
 
       - name: Restore dependencies
         run: |