Remove GitHub action .NET Core side by side hacks

It is causing this error:

```
Run SET DOTNET_31_ROOT=%DOTNET_ROOT:3.1.300=2.1.806%
  SET DOTNET_31_ROOT=%DOTNET_ROOT:3.1.300=2.1.806%
  xcopy /s /y /d %DOTNET_31_ROOT% %DOTNET_ROOT%
  shell: C:\windows\system32\cmd.EXE /D /E:ON /V:OFF /S /C "CALL "{0}""
  env:
    DOTNET_ROOT: C:\Users\runneradmin\AppData\Local\Microsoft\dotnet
Cannot perform a cyclic copy
0 File(s) copied
Error: Process completed with exit code 4.
```

Moreover, this workaround should no longer be required, see actions/setup-dotnet#25

.github/workflows/build.yaml

@@ -29,15 +29,6 @@ jobs:
         uses: actions/[email protected]
         with:
           dotnet-version: 3.1.300
-      - name: .net SxS Unix
-        if: matrix.os != 'windows-latest'
-        run: rsync -a ${DOTNET_ROOT/3.1.300/2.1.806}/* $DOTNET_ROOT/
-      - name: .net Sxs Windows
-        if: matrix.os == 'windows-latest'
-        run: |
-          SET DOTNET_31_ROOT=%DOTNET_ROOT:3.1.300=2.1.806%
-          xcopy /s /y /d %DOTNET_31_ROOT% %DOTNET_ROOT%
-        shell: cmd
       - name: Build with cake
         uses: cake-build/[email protected]
         with: