Draft: Support --vary=build_path.path=/build/path

This series adds support for making the build_path variation use a specified path.

This would allow using --control-build=/path/to/debian.org/debs/ --vary=build_path.path=/build/reproducible-path/PACKAGE-VERSION like used on buildd.debian.org, allowing reprotest to test builds with matching build paths, while still searching for other reproducibility issues.

This might also allow the salsa-ci jobs to use --control-build with --vary=build_path.path to avoid doing an extra reprotest build when build path variations are disabled.

Confusingly and somewhat ironically, this only changes the build path used when build_path variations are in play, the "control" build path is still the highly variable reprotest default. To make this affect the control build might require some significant changes to reprotest.

Starting this off as a draft request; the code is a little rough around the edges and needs some actual documentation.