GitHubCI: use env var for .NET version

This way it doesn't get repeated in each job.

Author: knocte

.github/workflows/CI.yml

@@ -4,6 +4,9 @@ on:
   push:
   pull_request:
 
+env:
+  DOTNET_VERSION: '6.0.428'
+
 jobs:
   buildAndTestWindows:
 
@@ -15,7 +18,7 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v3
       with:
-        dotnet-version: 6.0.428
+        dotnet-version: ${{ env.DOTNET_VERSION }}
     - name: Build & Run Tests
       run: |
         .\build.cmd
@@ -40,7 +43,7 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v3
       with:
-        dotnet-version: 6.0.428
+        dotnet-version: ${{ env.DOTNET_VERSION }}
     - name: Build & Run Tests
       run: |
         ./build.sh
@@ -65,7 +68,7 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v3
       with:
-        dotnet-version: 6.0.428
+        dotnet-version: ${{ env.DOTNET_VERSION }}
     - name: Build & Run Tests
       run: |
         ./build.sh