print-dev-env: stop inadvertently adding . to PATH

[lbodor]

Motivation

Ensure that . <(nix print-dev-env) correctly handles the case when PATH is empty.

Context

Fixes #8032 by changing the output of

nix print-dev-env nixpkgs#hello | grep nix_saved_

from

nix_saved_PATH="$PATH"
nix_saved_XDG_DATA_DIRS="$XDG_DATA_DIRS"
PATH="$PATH:$nix_saved_PATH"
XDG_DATA_DIRS="$XDG_DATA_DIRS:$nix_saved_XDG_DATA_DIRS"

to

nix_saved_PATH="$PATH"
nix_saved_XDG_DATA_DIRS="$XDG_DATA_DIRS"
PATH="$PATH${nix_saved_PATH:+:$nix_saved_PATH}"
XDG_DATA_DIRS="$XDG_DATA_DIRS${nix_saved_XDG_DATA_DIRS:+:$nix_saved_XDG_DATA_DIRS}"

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

Priorities

Add 👍 to pull requests you find important.

[roberth]

This looks uncontroversial. Please add test cases to tests/nix-shell.sh.
Looks like you'll want to change the existing test case (and your own) to run in a subshell

-source <(...
+(
+  source <(...

[lbodor]

I've added a test, thanks for the pointer. Please let me know what you think.

[edolstra]

No need for a separate scratch directory, just use $TEST_ROOT instead of $scratch.

[github-actions]

Backport failed for 2.11-maintenance, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally.

git fetch origin 2.11-maintenance
git worktree add -d .worktree/backport-8033-to-2.11-maintenance origin/2.11-maintenance
cd .worktree/backport-8033-to-2.11-maintenance
git checkout -b backport-8033-to-2.11-maintenance
ancref=$(git merge-base 208c8551249361d2383b44123b4ad156218c351b a3a6909bc8e33cf1bf102a315f398a252607b5e0)
git cherry-pick -x $ancref..a3a6909bc8e33cf1bf102a315f398a252607b5e0