Add a job to run 'uscan' and test if debian/watch works correctly

Closes #539 (closed)

Testing: I made tests with this repository that has a correct debian/watch:
https://salsa.debian.org/Thais-ra/galpy/-/pipelines/1057798
https://salsa.debian.org/Thais-ra/galpy/-/jobs/9346235

And here I changed debian/watch to force fail:
https://salsa.debian.org/Thais-ra/galpy/-/pipelines/1058456
https://salsa.debian.org/Thais-ra/galpy/-/jobs/9351226