Drop net6 forced install, it's built-in all agents now

Author: kzu

.github/workflows/build.yml

@@ -53,12 +53,6 @@ jobs:
           submodules: recursive
           fetch-depth: 0
 
-      - name: ⚙ dotnet
-        uses: actions/setup-dotnet@v1
-        if: matrix.os != 'windows-latest'
-        with:
-          dotnet-version: '6.0.x'
-
       - name: 🙏 build
         run: dotnet build -m:1 -p:VersionLabel="$GITHUB_REF.$GITHUB_RUN_NUMBER"