Ensure error messages don't leak private key (backport #11523)

[mergify]

Motivation

Since #8766, invalid base64 is rendered in errors, but we don't actually want to show this in the case of an invalid private keys.

Context

#8766

Priorities and Process

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This is an automatic backport of pull request #11523 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 2b6b03d has failed:

On branch mergify/bp/2.24-maintenance/pr-11523
Your branch is ahead of 'origin/2.24-maintenance' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 2b6b03d8d.
  (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/libfetchers/git-utils.cc
	modified:   src/libstore/machines.cc
	modified:   src/libstore/ssh.cc
	modified:   src/libstore/ssh.hh
	modified:   src/libutil/hash.cc
	modified:   src/libutil/signature/local-keys.hh
	modified:   src/libutil/util.cc
	modified:   src/libutil/util.hh
	modified:   tests/unit/libexpr/nix_api_expr.cc

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/libutil/signature/local-keys.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

[Ericson2314]

Needs backport of #11390. Made those, now waiting.

[Mic92]

Backport without conflicts in #11523