[Bootstrapper] Decide Default Install Location

[nagilson]

📓 Summary

Global.json parsing is handled in #354, but what if a global.json and user option does not tell us where to install to? We should have some sort of 'default' installation directory.

Additional Context

Starting options are a folder in temp (likely protected) or in the homedir.

Blocked Until These Issues Are Done:

[nagilson]

Based on dotnet/sdk#48893, HOME is not a suitable place to put a global installation directory for Mac.

/home/user/.blah is used by nvm. I guess 'user global' is the way we're going here, so that's ok.

[nagilson]

Decision from meeting:
Installation Directory Default for when you run '__ install':

1. A configuration option provided via the CLI (-d --directory or -o –output), etc.

2. Look at the first path global.json setting: ('sdk' : 'paths': [])
   The Host already supports this. Supportinglocal SDK deployment in global.json by jaredpar · Pull Request Add support for workloads as part of the uninstall tool #303 · dotnet/designs
   "sdk" : { "paths: [".dotnet", "$host$"], "version": "8.0.200", "rollForward": "latestFeature" }

3. Utilize the location of the existing muxer + a folder suffix.

4. A default location, app data/roaming/, or XGD_DIR/HOME directory on unix.
   Suffix: .dotnet + /bootstrapper/v1

[nagilson]

We also need to figure out where to put the bootstrapper itself, and where to update it (temp folder mutex like vscode), and if we should have pinning of self update versions.