Fix Shellcheck SC2016: Single quotes don't expand

Expressions don't expand in single quotes, use double quotes for that.

https://github.com/koalaman/shellcheck/wiki/SC2016



This specifically warned about the '$aregis...' part of the sed script.
Which is a relatively obscure use of sed.

Splitting this into two commands makes its intent more obvious and
avoids contravening Shellcheck's lints.

Signed-off-by: Dan Callahan <danc@element.io>