GitHubCI: tweak linux*-github lanes to setup .NET6

Same as some macOS&windows lanes already do, otherwise we get
this error saying that the oldest .NET available is 8.x:

```
You must install or update .NET to run this application.

App: /home/runner/work/geewallet/geewallet/src/GWallet.Frontend.Console/bin/Debug/net6.0/GWallet.Frontend.Console.dll
Architecture: x64
Framework: 'Microsoft.NETCore.App', version '6.0.0' (x64)
.NET location: /usr/share/dotnet/

The following frameworks were found:
  8.0.6 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]
  8.0.22 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]
  9.0.6 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]
  9.0.11 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]
  10.0.0 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]

Learn more:
https://aka.ms/dotnet/app-launch-failed
```

Author: knocte

.github/workflows/CI.yml

@@ -220,6 +220,10 @@ jobs:
     - uses: actions/checkout@v1
       with:
         submodules: false
+    - name: Setup .NET SDK 6.0.x
+      uses: actions/[email protected]
+      with:
+        dotnet-version: '6.0.113'
     - name: install missing dependencies
       run: sudo DEBIAN_FRONTEND=noninteractive apt install --yes fsharp nunit-console
     - name: check runtime version(s)
@@ -245,6 +249,10 @@ jobs:
     - uses: actions/checkout@v1
       with:
         submodules: false
+    - name: Setup .NET SDK 6.0.x
+      uses: actions/[email protected]
+      with:
+        dotnet-version: '6.0.113'
     - name: install missing dependencies
       run: sudo DEBIAN_FRONTEND=noninteractive apt install --yes fsharp nunit-console
     - name: install last version of mono (Microsoft APT repositories)
@@ -475,6 +483,10 @@ jobs:
     - uses: actions/checkout@v1
       with:
         submodules: false
+    - name: Setup .NET SDK 6.0.x
+      uses: actions/[email protected]
+      with:
+        dotnet-version: '6.0.113'
     - name: install missing dependencies
       run: sudo DEBIAN_FRONTEND=noninteractive apt install --yes fsharp nunit-console
     - name: check runtime version(s)
@@ -498,6 +510,10 @@ jobs:
     runs-on: ubuntu-22.04
     steps:
     - uses: actions/checkout@v1
+    - name: Setup .NET SDK 6.0.x
+      uses: actions/[email protected]
+      with:
+        dotnet-version: '6.0.113'
     - name: install missing dependencies
       run: sudo DEBIAN_FRONTEND=noninteractive apt install --yes fsharp nunit-console
     - name: install last version of mono (Microsoft APT repositories)