libutil: Less unnecessary copying in LRUCache (backport #13193)

[mergify]

Motivation

There's some low hanging fruit in terms of code simplification and getting rid
of unnecessary copying (see the second commit). Also moving the cache entry to the back of the lru list can be
made much cheaper by just using splice method, which just reassigns some pointers in the linked list (having pointer stability is the main purpose of std::list after all).

Context

Some unified caching primitives are useful to reuse in other places (e.g. #13137 (comment))

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This is an automatic backport of pull request #13193 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 90d70aa has failed:

On branch mergify/bp/2.28-maintenance/pr-13193
Your branch is up to date with 'origin/2.28-maintenance'.

You are currently cherry-picking commit 90d70aa4c.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   maintainers/flake-module.nix

no changes added to commit (use "git add" and/or "git commit -a")

Cherry-pick of cd61e92 has failed:

On branch mergify/bp/2.28-maintenance/pr-13193
Your branch is ahead of 'origin/2.28-maintenance' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit cd61e922f.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   src/libutil/include/nix/util/lru-cache.hh

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/libstore/store-api.cc

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally