mkarchroot: Don't let the environment affect pacstrap (sans proxy settings).

A previous iteration of this change (libretools commit d7dcce53396d)
simply inserted `env -i` to clear the environment.

However, that lead to it ignoring proxy settings, which some users had
problems with:

    https://labs.parabola.nu/issues/487:
    > To fix other bugs, the pacstrap environment is blank, which also
    > means that the proxy settings are blank.

So (in libretools commit d17d1d82349f), I changed it to use `declare
-x` to inspect the environment, and create a version of it only
consisting of variables ending with "_proxy" (case-insensitive).

I honestly don't remember what "other bugs" prompted me to clear the
environment in the first place.