Fix deadlock on joins cache

[elpaso]

Fixes #43287

The critical section was called recursively by a call from the model
connected to joinedFieldsChanged.

By moving the mutex into a scope we can safely release it before
emitting the signal.

[elpaso]

Ping @nyalldawson maybe related to #43427 ?

[nyalldawson]

Ping @nyalldawson maybe related to #43427 ?

Maybe the other way round, and #43427 wasn't helping here. But the issue fixed by #43427 wasn't happening as a result of modifying joins.

[qgis-bot]

The backport to release-3_16 failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release-3_16 release-3_16
# Navigate to the new working tree
cd .worktrees/backport-release-3_16
# Create a new branch
git switch --create backport-43434-to-release-3_16
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 6db7e2fb8d6b7d9364a1070b10e5d7741d5eee10
# Push it to GitHub
git push --set-upstream origin backport-43434-to-release-3_16
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-3_16

Then, create a pull request where the base branch is release-3_16 and the compare/head branch is backport-43434-to-release-3_16.