nix-profile.sh.in: fix envvar condition (backport #11541)

[mergify]

Motivation

#11351 introduced a regression. The check for the existence of an environment variable caused an error for some shell versions.

Context

Should fix #11535.

Priorities and Process

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This is an automatic backport of pull request #11541 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 97fffd8 has failed:

On branch mergify/bp/2.24-maintenance/pr-11541
Your branch is up to date with 'origin/2.24-maintenance'.

You are currently cherry-picking commit 97fffd876.
  (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:
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	both modified:   scripts/nix-profile.sh.in

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

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