Don't use backticks for command substitution

While setting up a mirror, an unexperienced sysadmin replaced:

# MIRRORNAME=`hostname -f`

With the following:

MIRRORNAME=`foo.example.org`

The sysadmin is Korean and doesn't seem to be familiar with English, so it's very possible that they didn't even see it was a backtick, or maybe they're not familiar with Bash and thought that quotes and backticks were the same.

So let's switch to modern syntax to avoid this kind of mistake in the future.