Github Actions: make all dotnet commands work by adding the sln file

* previously one got errors like this:

MSBUILD : error MSB1003: Specify a project or solution file.
The current working directory does not contain a project or solution file.

Author: janusw

.github/workflows/main.yml

@@ -20,8 +20,8 @@ jobs:
       with:
         dotnet-version: 3.1.301
     - name: Install dependencies
-      run: dotnet restore
+      run: dotnet restore src/GeoJSON.Net.sln
     - name: Build
-      run: dotnet build --configuration Release --no-restore
+      run: dotnet build src/GeoJSON.Net.sln --configuration Release --no-restore
     - name: Test
-      run: dotnet test --no-restore --verbosity normal
+      run: dotnet test src/GeoJSON.Net.sln --no-restore --verbosity normal