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

[JanProvaznik]

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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.

[JanProvaznik]

Internal discussion: Devkit can work around this faster if they need it (and install-scripts can't deploy this month if it's not critical).

There is no silver bullet for figuring out if the environment is set up correctly after the install scripts run and if a reinstall would help. But checking that the dotnet.exe/dotnet exists is a good idea with low priority.