Add support for Nushell env var settings

[MaxDesiatov]

Currently, swiftly incorrectly generates Bash env var scripts when it is installed in Nushell.

[cmcgee1024]

Overall, this looks good.

question: where does the nu profile get modified so that when you start a new nu login shell you get the updated PATH and environment variables? For example, with bash/zsh it typically adds a source line in ~/.profile or ~/.zprofile that sources the env file in the swiftly home directory.

suggestion: The documentation and readme could be updated to announce this new feature of swiftly.

[cmcgee1024]

@swift-ci test macOS

[cmcgee1024]

@MaxDesiatov I was attempting to just make some suggested changes and ended up updating the branch. Apologies. I think that the merge conflicts are resolved, and things are passing the checks now.

[cmcgee1024]

It looks like ubuntu doesn't yet come with nu shell, so there will likely need to be a release check similar to the other shell-based checks in .github/workflows/pull_request.yml, except it could manually curl the nu binary from a release on GitHub here: https://github.com/nushell/nushell/releases, install the binary, chsh to that shell and do the verification.