Per review feedback on #1745: rather than adding one boolean option per
HOMEBREW_NO_* env var, expose a generic onActivation.extraEnv attrset that
is prepended to brewBundleCmd alongside HOMEBREW_NO_AUTO_UPDATE=1. This
covers the same use case and the long tail of other HOMEBREW_NO_* vars
without tying the module to Homebrew's naming choices.
The global-scope booleans are dropped with no replacement — users who want
these variables set globally can write them directly into
environment.variables, so a global.extraEnv would be pure sugar.
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