Add ability to install just the runtime, rather than the SDK

[thomaslevesque]

Description:
This action sets up the .NET SDK, but in some cases I'd like to install just a specific runtime version, not the whole SDK.

Justification:
I have a library with tests for multiple .NET versions. I want to use the latest SDK, but to be able to run the tests for all the supported .NET versions, I also need the appropriate runtime versions to be installed.

Are you willing to submit a PR?
Open to it, but would need some guidance, since I'm not familiar with the code.

Note: this seems to be supported by the install.sh and install.ps1 scripts, but I see no way to pass the appropriate parameters to the script when using the action. Am I missing something?

[IvanZosimov]

Hi, @thomaslevesque 👋 Thank you for this feature request! We will investigate the possibility of adding this feature and get back to you with updates.

[ch1seL]

absolutely support, the most common case when building multitarget projects

[VAllens]

This switch is cool, but I have another opposite idea, add a sdk-only switch.
Right now by default the runtime is always installed first, then the sdk.
When one wishes to install sdk only, there's really no need to install runtime separately before, since sdk already includes runtime.