Support BROWSER env variable when selecting a browser to launch for oauth2

[sam-mfb]

Description

The current code for launching a browser to do oauth2 login in Linux hardcodes three linux browser launchers.

providers := []string{"xdg-open", "x-www-browser", "www-browser"}

vendor/github.com/pkg/browser/browser_linux.go

It would be nice if that choice could be overridden via the standard BROWSER= env variable. This would allow the end user to provide alternative browser launch commands. This is, for example, what the github browser utility does go-gh/blob/trunk/pkg/browser/browser.go

The use case for this is users running in dev containers. This allows them to have custom scripts that forward the browser request to outside of the dev container so it can be launched from the host browser (since the dev container likely does not contain a gui browser)

[sam-mfb]

Note that if xdg-open is installed, IT respects the BROWSER env variable, so arguably the solution is just to install xdg-open. I defer to the maintainers on whether to close this ticket in favor of that solution, i.e., just install xdg-open...