Build: set CANCELLED state when the build is cancelled (#11171)
* Build: set `CANCELLED` state when the build is cancelled
This bug was introduced in
https://github.com/readthedocs/readthedocs.org/pull/10922/files#diff-4eae6664ca2124d124dbfe04a8000483c7a23d426c3f390721d6e80fe5891064L504
by mistake.
Closes #11170
* Lint
* Test for `CANCELLED_BY_USER`