Reproducible builds of opam itself

[dra27]

Some good progress has been made here since 2016. Opam 2.0.3 was reproducible. Unfortunately, some changes introduced in later versions broke this nice property. I have tried to make it reproducible. During the build process, it turns out that setting HOME variable to some static setting and using this patch [1] makes most of the issues to go away (they appear only in manpages, which contain paths).

[1] https://sources.debian.org/src/opam/2.0.8-1/debian/patches/0004-Use-HOME-env-variable-instead-of.patch/

It would great if this patch (or a variation of it) is integrated in opam's repo 🙏🏼

There is still one issue though which can be observed here: https://tests.reproducible-builds.org/debian/rb-pkg/unstable/amd64/diffoscope-results/opam.html and which affects opam-admin-cache.1

I didn't want to heavily change the code or the behavior. What would be your recommendation to fix this last issue?

Originally posted by @mehdid in #2720 (comment)

[dra27]

@avsm notes in #2720 (comment):

It looks like opam admin cache embeds the cwd in the man page:

       DIR (absent=~/src/git/ocurrent/ocaml-ci/cache)

...we should just change this in the manual page generation to be "absent=" as specifying the exact directory is inaccurate in the event the manual page is saved.