Pin dotnet version for GHA formatting (#19)

Pins dotnet version for dotnet-format

Author: Henry Peteet

.github/workflows/pre-commit.yml

@@ -22,7 +22,7 @@ jobs:
         ruby-version: '2.6'
     - uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: '6.0.x'
+        dotnet-version: '6.0.100'
         include-prerelease: true
     - name: Install manual dependencies
       run: |