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>

debian/test/provision.sh

@@ -19,5 +19,6 @@ EOF
 
 dpkg -i "$deb"
 
-sed -i -e '/port: 8...$/{s/8448/18448/; s/8008/18008/}' -e '$aregistration_shared_secret: secret' /etc/matrix-synapse/homeserver.yaml
+sed -i -e 's/port: 8448$/port: 18448/; s/port: 8008$/port: 18008' /etc/matrix-synapse/homeserver.yaml
+echo 'registration_shared_secret: secret' >> /etc/matrix-synapse/homeserver.yaml
 systemctl restart matrix-synapse