Autodetect the RELEASE from d/changelog

Closes: #186 (closed)

this PR checks the distribution as set in d/changelog, and uses this to guess a value for RELEASE (this is done in the extract-sources job)

• if RELEASE is already set (e.g. via project variables), it shouldn't do anything
• if the target distribution is UNRELEASED, it uses unstable
• if the target distribution is UNRELEASED-<something>, it uses <something>
• if the target distribution is <something>-UNRELEASED, it uses <something>
• if the target distribution is <something>, it uses <something>

here's a few test-runs:

• d/changelog set to unstable
• d/changelog set to experimental
• d/changelog set to bullseye-backports

(i hope i got the links right, they all look the same to me).

most things seem to work, except for the lintian job. afaict this is a bug in gitlab, where dotenv variables cannot be used in jobs that declare a needs artifact. (i saw that bug-report today, but have lost it for now).