Add 'LANG' to the environment filter to ensure it passes into brew if/when set in the user's environment

[jamesrtnz]

As per https://github.com/orgs/Homebrew/discussions/6785; currently, the end user's 'LANG' environment variable isn't included in the filter, so is dropped when 'brew' commands are run.

The knock-on impact of this is that the glibc 'postinstall' code which attempts to include the user's language support doesn't see any values for LANG, so they don't get included automatically.

This PR will resolve that; and will mean that future glibc updates (on Linux particularly) will include other languages besides en_US.UTF-8 if the end-user is using them.

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[MikeMcQuaid]

Thanks!

Let's use one of the other bin/brew paths here and pass it through as HOMEBREW_LANG where it can specifically be set in the glibc postinstall code rather than being applied universally in all Homebrew formulae, casks and core code where it is (much) more likely to break things.

[jamesrtnz]

OK; if I understand the codebase, that will be an update to env_config.rb, and also an update to the glibc formula?

[MikeMcQuaid]

@jamesrtnz add to https://github.com/jamesrtnz/brew/blob/4ec9c96e2e97ed10684dc98d91e9f9ac71b73d1e/bin/brew#L215-L239 and then to formula

[MikeMcQuaid]

that will set HOMEBREW_LANG from LANG automatically