Add an EXTRACOMPONENTS envvar

This is so if you need additional components for your build, they can be made available. For example, if you wanted thirdparty/ceph-reef available, you would set EXTRACOMPONENTS to thirdparty/ceph-reef in CI.

This works by setting up an apt .sources file that looks like:

Types: deb
URIs: http://apt.wikimedia.org/wikimedia
Suites: ${SUITE}-wikimedia
Signed-By: /etc/apt/keyrings/wikimedia-archive-keyring.gpg
Components: ${EXTRACOMPONENTS}