When we're automatically setting `HOMEBREW_NO_INSTALL_FROM_API`
when on an old macOS version or with a non-default prefix (e.g. cases
where you're going to be mostly building from source).
My initial plan was to set
`HOMEBREW_AUTOMATICALLY_SET_NO_INSTALL_FROM_API` in these cases but
it's used differently enough it made sense to add another internal
variable instead: `HOMEBREW_INSTALL_FROM_API_UNSUPPORTED`.
At the moment this is only used to avoid printing the "You have set
`HOMEBREW_NO_INSTALL_FROM_API`" message inside `brew update` but may
make sense to use in other places over time.
Ideally, we'll get rid of these automatic sets of
`HOMEBREW_NO_INSTALL_FROM_API` and perhaps even the variable entirely.
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