GithubCI: set rollForward in linux-newLTS-github*

Set rollForward to major in linux-newLTS-github-* jobs to avoid
errors like:
```
Testhost process for source(s) '/home/runner/work/fsx/fsx/Fsdk.Tests/bin/Debug/net6.0/Fsdk.Tests.dll' exited with error: You must install or update .NET to run this application.
App: /home/runner/work/fsx/fsx/Fsdk.Tests/bin/Debug/net6.0/testhost.dll
Architecture: x64
Framework: 'Microsoft.NETCore.App', version '6.0.0' (x64)
.NET location: /usr/share/dotnet/
The following frameworks were found:
  8.0.19 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]
  9.0.4 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]
```

Author: webwarrior-ws

.github/workflows/CI.yml

@@ -144,6 +144,8 @@ jobs:
 
   linux-newLTS-github--dotnet-and-xbuild:
     runs-on: ubuntu-22.04
+    env:
+      DOTNET_ROLL_FORWARD: major
     steps:
     - uses: actions/checkout@v1
     - name: check mono version
@@ -175,6 +177,8 @@ jobs:
 
   linux-newLTS-github--dotnet-and-msbuild:
     runs-on: ubuntu-22.04
+    env:
+      DOTNET_ROLL_FORWARD: major
     steps:
     - uses: actions/checkout@v1
     - name: check mono version