Make sure our extra version information doesn't get passed to gi-docgen

gi-docgen uses a python library for verifing version numbers and it's quite
strict. So we just rebuild our version number from the parts, skipping the
extra bit and it's happy with that.

Testing Done:
Ran the `fedora-rawhide-amd64` convey plan without issue.

Reviewed at https://reviews.imfreedom.org/r/2899/