Fix aggregate function returns incorrect results

[nyalldawson]

When the aggregate uses variables, we need to ensure that the
cache key correctly considers the current value of ALL those
variables. Otherwise we'll return incorrect results when
an expression is re-evaluated after changing the variable
value for the context.

Fixes use of aggregate function with @symbol_label in legends.

Fixes #58221

(Possibly also fixes #58315, but untested)

[github-actions]

🪟 Windows builds

Download Windows builds of this PR for testing.
Debug symbols for this build are available here.
(Built from commit 8c70a83)

🪟 Windows Qt6 builds

Download Windows Qt6 builds of this PR for testing.
(Built from commit 8c70a83)

[qgis-bot]

The backport to queued_ltr_backports failed:

The process '/usr/bin/git' failed with exit code 1

stderr

error: could not apply 7b88103128d... Fix aggregate function returns incorrect results
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config advice.mergeConflict false"

stdout

Auto-merging python/PyQt6/core/auto_generated/qgsexpressioncontext.sip.in
Auto-merging python/core/auto_generated/qgsexpressioncontext.sip.in
Auto-merging src/core/expression/qgsexpressionfunction.cpp
CONFLICT (content): Merge conflict in src/core/expression/qgsexpressionfunction.cpp
Auto-merging src/core/qgsexpressioncontext.cpp
Auto-merging src/core/qgsexpressioncontext.h
Auto-merging tests/src/core/testqgsexpression.cpp
Auto-merging tests/src/core/testqgsexpressioncontext.cpp

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-queued_ltr_backports queued_ltr_backports
# Navigate to the new working tree
cd .worktrees/backport-queued_ltr_backports
# Create a new branch
git switch --create backport-59000-to-queued_ltr_backports
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 7b88103128db0cc33c5718ad273ddf51cb192bf9,8c70a83935423fa9aac8ef37651b92de5192c128
# Push it to GitHub
git push --set-upstream origin backport-59000-to-queued_ltr_backports
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-queued_ltr_backports

Then, create a pull request where the base branch is queued_ltr_backports and the compare/head branch is backport-59000-to-queued_ltr_backports.