dockerTools.buildImageWithNixDb: simplifications

[nlewo]

Since Nix 2 is now the stable Nix version, we can use closureInfo
which simplifies the Nix database initialisation (size and hash are
included in the "dump").

Motivation for this change

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
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• 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.

---

[nlewo]

@GrahamcOfBorg test docker-tools

[GrahamcOfBorg]

Success on aarch64-linux

Attempted: tests.docker-tools

No partial log is available.

[GrahamcOfBorg]

Success on x86_64-linux (full log)

Attempted: tests.docker-tools

Partial log (click to expand)

docker: exit status 1
syncing
docker: running command: sync
docker: exit status 0
test script finished in 134.22s
cleaning up
killing docker (pid 593)
vde_switch: EOF on stdin, cleaning up and exiting
vde_switch: Could not remove ctl dir '/build/vde1.ctl': Directory not empty
/nix/store/rn4xz5m2mbrzwqvvwhy766hbyccdjnbg-vm-test-run-docker-tools

[grahamc]

Unfortunately we shouldn't depend upon 2.0 yet.

[nlewo]

@grahamc hmm, I'm surprised since VM build tools already depend of 2.0.
But I could resubmit this PR later since this is just to clean and simplify code base. Do you know when we could depend of 2.0?