refactor: Extract EvalState::realiseString (backport #12362)

[mergify]

Motivation

This is an operation that's needed in a couple of places for

• Substitute downstream placeholders when realising a "string with context" #11263

Context

We'll want something similar for path coercions. Scoped out for this one.

This is a trivial refactor to support fixes, so I propose to backport this.

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This is an automatic backport of pull request #12362 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 7465fbe has failed:

On branch mergify/bp/2.24-maintenance/pr-12362
Your branch is up to date with 'origin/2.24-maintenance'.

You are currently cherry-picking commit 7465fbe92.
  (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/libexpr-c/nix_api_value.cc
	modified:   src/libexpr/eval.hh

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/libexpr/primops.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