Fix `excluded_paths_regex` in `finalize_manifest.py`

[woju]

This is split from #120.

At least some of /var needs to be preserved, /run needs to be discarded. Very likely other paths.

References:

• https://manpages.debian.org/testing/manpages/hier.7.en.html
• https://manpages.debian.org/testing/systemd/file-hierarchy.7.en.html
• https://refspecs.linuxfoundation.org/FHS_3.0/fhs/index.html
• https://www.debian.org/doc/debian-policy/ch-opersys.html#file-system-hierarchy
• https://docs.fedoraproject.org/en-US/packaging-guidelines/#_filesystem_layout