feat(install): Add PIXI_NO_PATH_UPDATE for PATH update suppression

[chawyehsu]

Close #686

The PR mainly adds the option of using a PIXI_NO_PATH_UPDATE environment variable to suppress PATH update, when installing pixi from install script.

Besides the option added, since the PR is about environment variable, the way of updating PATH in the PowerShell install script has also been modified from using [Environment]::SetEnvironmentVariable to set environment variable to get and update environment variable from Windows registry.

This change doesn't affect pixi installation directly and is primarily to address a potential issue that SetEnvironmentVariable expands all environment variables and this behavior may not be welcomed in some cases.

$Env:PATH
#> %USERPROFILE%\bin;

# when using `SetEnvironmentVariable` to update PATH, `%USERPROFILE%` will be expanded
#> C:\Users\userA\bin;C:\pixi\bin

# when using registry to update PATH, `%USERPROFILE%` will be kept unexpanded
#> %USERPROFILE%\bin;C:\pixi\bin

Related code are copied from ScoopInstaller/Install I'm maintaining.

[ruben-arts]

Thanks!

[ruben-arts]

Cool wasn't aware you could do this!

[mariusvniekerk]

This PR breaks the installer for linux in some cases where it fails with

bash: line 83: PIXI_NO_PATH_UPDATE: unbound variable