Adds build_path checks for deb pkg reprotest

[conorsch]

In #231 we noticed that the filesystem path used to build the debian
packages was breaking reproducibility. That's due to the filepath being
recorded inside direct_url.json, as stipulated by PEP610.

Also relevant is PEP427, which describes the RECORD file for wheels.
Here we remove that file, as well, to ensure full reproducibility
regardless of path.

Testing

To evaluate change in the problematic behavior reported in #231, clone this branch fresh in a few different locations, e.g. in DispVMs, and try rebuilding. Make sure that the absolute path of the repo is different between builds! The checksum of the .deb packages should be identical.

[kushaldas]

In my system the tests are failing for a different reason:

 make[1]: Warning: File 'debian/rules' has modification time 206 s in the future
make[1]: warning:  Clock skew detected.  Your build may be incomplete.
 debian/rules binary
make[1]: Warning: File 'debian/rules' has modification time 206 s in the future
make[2]: Warning: File 'debian/rules' has modification time 206 s in the future
/root/code/securedrop-debian-packaging/.venv/lib/python3.7/site-packages/virtualenv.py:1047: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
  import imp
make[2]: warning:  Clock skew detected.  Your build may be incomplete.
make[2]: Warning: File 'debian/rules' has modification time 203 s in the future
make[2]: warning:  Clock skew detected.  Your build may be incomplete.
dpkg-gencontrol: warning: Depends field of package securedrop-export: substitution variable ${python3:Depends} used, but is not defined
make[1]: warning:  Clock skew detected.  Your build may be incomplete.
 dpkg-genbuildinfo
 dpkg-genchanges  >../securedrop-export_0.2.6+buster_amd64.changes
dpkg-genchanges: info: including full source code in upload
 dpkg-source --after-build .

[conorsch]

CI was previously failing because I'd omitted a debian/rules update for the export package. Added, and now CI is passing. I've added a test plan, marking as ready for review.

[sssoleileraaa]

lgtm: works with different build roots (repo paths) and across different machines (also learned about being able to specify PKG_GITREF=main from conor)