This updates commit 11ab545f3 "install: Set the locale ..."
to be somewhat cleaner, and to explain more in the commit message.
In some environments, either pip itself fails or some packages fail to
install, and setting the locale to en_US.UTF-8 resolves the issue.
We heard reports of this kind of behavior with at least two different
sets of symptoms, with 1.7.0 or its release candidates:
https://chat.zulip.org/#narrow/stream/general/subject/Trusty.201.2E7.20Upgrade/near/302214https://chat.zulip.org/#narrow/stream/production.20help/subject/1.2E6.20to.201.2E7/near/306250
In all reported cases, commit 11ab545f3 or equivalent fixed the issue.
Setting LC_CTYPE is redundant when also setting LC_ALL, because LC_ALL
overrides all `LC_*` environment variables; so skip that. Also move
the line in `install` to a more appropriate spot, and adjust the
comments.
64c608a51a