check if nightly is outdated

[johnnychen94]

If registered mirror stops those nightly builds, there will be cases that an old version is installed. This won't be a big issue since we could always get updated nightly builds from upstream Official.

[johnnychen94]

A simple solution to this is to check the commit date and throw warnings if it's too old:

julia> Base.GIT_VERSION_INFO.date_string
"2021-08-11 14:37 UTC"

Thus for example, if jill install latest gives a binary that is more than 7 days old, print a warning on it.

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Of course, if we hardcode and always point "latest" to "Official" then this won't be an issue at all.