cmd/shellenv: set XDG_DATA_DIRS on Linux

[notfirefox]

Some programs like vapigen might not work correctly, when XDG_DATA_DIRS does not include $HOMEBREW_PREFIX/share. Instead of requiring the user to manually adjust the shell environment, we can set XDG_DATA_DIRS as part of cmd/shellenv, so that it is sufficient to run brew shellenv on any particular system.

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Fixes #18317

[carlocab]

Seems reasonable to me, though I don't know how to review the PowerShell code.

Also wondering if we should just set this unconditionally on Linux.

[notfirefox]

Also wondering if we should just set this unconditionally on Linux.

I think that should be fine. Although we should consider the following from the XDG Base Directory Specification.

If $XDG_DATA_DIRS is either not set or empty, a value equal to /usr/local/share/:/usr/share/ should be used.

Therefore we might want to do something like this:

export XDG_DATA_DIRS="${HOMEBREW_PREFIX}/share:${XDG_DATA_DIRS:-/usr/local/share:/usr/share}"

[carlocab]

I think that should be fine. Although we should consider the following from the XDG Base Directory Specification.

Yes, we should follow that if we set it, but that doesn't seem to be much simpler than what we're already doing now. Fine with leaving it as is if it's enough to silence the warning from brew doctor.

[carlocab]

Thanks!

[MikeMcQuaid]

Thanks again @notfirefox!