releaseTools.debBuild: split checkinstall into separate phase

[abbradar]

Motivation for this change

This is needed for my incoming pull request to Nix that improves situation with Debian packages.

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[mention-bot]

@abbradar, thanks for your PR! By analyzing the history of the files in this pull request, we identified @edolstra, @rbvermaa and @Profpatsch to be potential reviewers.

[abbradar]

This breaks compatibility with Nix's release.nix because postInstall now runs after make install and not after a resulting Debian package is installed (which seems more "right" to me). However, Nix's corresponging PR is still not merged so Nix .deb packages are now broken. We may want a revert -- not sure how to proceed.

[7c6f434c]

Hm. Didn't understand there is a circular dependency between these PRs… The description looked like a single-direction dependency. And merging something into Nix will probably take some time…

I guess then reverting for now could be a reasonable thing to do…

[abbradar]

Yep, I did a bad job describing the change (also didn't mention the incompatibility). I'll revert it, sorry for the trouble!