GitHub Actions: use .NET 7 SDK for .NET 6 build

Our `PublicApi` verification test passes on AppVeyor while it fails on
GitHub Actions. One difference between the two is that AppVeyor uses v7
of the .NET SDK, while GitHub Actions uses v6.

Installing both .NET 6 and 7 will hopefully resolve the `PublicApi` test
failure with GitHub Actions builds.

Author: stakx

.github/workflows/build.yml

@@ -35,13 +35,18 @@ jobs:
         with:
           dotnet-version: 3.1.x
 
-      # Building requires an up-to-date .NET SDK.
-
       - name: Install .NET 6.0
         uses: actions/setup-dotnet@v1
         with:
           dotnet-version: 6.0.x
 
+      # Building requires an up-to-date .NET SDK.
+
+      - name: Install .NET 7.0
+        uses: actions/setup-dotnet@v1
+        with:
+          dotnet-version: 7.0.x
+
       # -----
       # Build
       # -----