Add docs for supporting multiple user install on windows

[dfordivam]

I tried these instructions by creating multiple user accounts.
It seems that settings the PATH is sufficient for the standard C:\ghcup installations.

For non-standard installation GHCUP_INSTALL_BASE_PREFIX is necessary, to ensure that the ghcup exe does not create / write to C:\ghcup

I am not sure how the GHCUP_MSYS2 + cabal related instructions are to be tested.

Fixes #1070

[hasufell]

On windows, we are multi user by default.

They need to adjust the path globally and not per user.

We do the PATH adjustment here:

ghcup-hs/scripts/bootstrap/bootstrap-haskell.ps1

Line 660 in e38c41b

Add-EnvPath -Path ('{0}\bin' -f ([System.IO.Path]::GetFullPath("$GhcupDir"))) -Container 'User'

So a global install could be supported by the powershell script trivially.

[dfordivam]

Ok, but this didn't work smoothly if the ghcup is not installed at 'C:/'.

It is possible to set the env var globally during install
[Environment]::SetEnvironmentVariable("GHCUP_INSTALL_BASE_PREFIX", 'C:\somedir', [System.EnvironmentVariableTarget]::Machine)

But if the GHCUP_INSTALL_BASE_PREFIX is not defined as user's env variable, then ghcup continues to work with C:\ prefix.

[hasufell]

But if the GHCUP_INSTALL_BASE_PREFIX is not defined as user's env variable, then ghcup continues to work with C:\ prefix.

That would appear to be a bug, no?

[dfordivam]

Yeah, likely an issue with lookupEnv. I looked around but couldn't find any more info about this issue. Or perhaps lookupEnv is fine, and this should be some other windows specific API?

[hasufell]

https://learn.microsoft.com/en-us/windows/win32/api/processenv/nf-processenv-getenvironmentvariablew

This function can retrieve either a system environment variable or a user environment variable.

[dfordivam]

Ok, after more debugging figured out the issue, the powershell needs to be restarted for all users after the install to load the new vars from global.

[dfordivam]

I am doing a rewrite of this to be clearer