Install scripts don't attempt to install when the runtime specific binaries are present but not the `dotnet` executable

https://github.com/dotnet/vscode-dotnet-runtime/issues/2059#issuecomment-2516628176

Install scripts could check that dotnet.exe / dotnet is present in the directory before deciding whether to install or report that it's already installed.

```
ls idir
LICENSE.txt ThirdPartyNotices.txt dotnet host shared
./dotnet-install.sh --install-dir idir/ --runtime aspnetcore
dotnet-install: ASP.NET Core Runtime with version '8.0.11' is already installed.

rm idir/dotnet
./dotnet-install.sh --install-dir idir/ --runtime aspnetcore
dotnet-install: ASP.NET Core Runtime with version '8.0.11' is already installed.
```

This change would enable a recovery from a corrupted install by using the same command again.