fix: set check level correctly during workflow (#5344)

Fixes a workflow bug where the `check-level` was not always set
correctly. Arguments to a `gh` call used to determine the `check_level`
were accidentally outside of the relevant command substitution (`$(gh
...)`).

-----

This can be observed in [these
logs](https://github.com/leanprover/lean4/actions/runs/10859763037/job/30139540920),
where the check level (shown first under "configure build matrix") is
`2`, but the PR does not have the `release-ci` tag. As a "test", run the
script for "set check level" printed in those logs (with some lines
omitted):
```
check_level=0

labels="$(gh api repos/leanprover/lean4/pulls/5343) --jq '.labels'"
if echo "$labels" | grep -q "release-ci"; then
  check_level=2
elif echo "$labels" | grep -q "merge-ci"; then
  check_level=1
fi

echo "check_level=$check_level"
```
Note that this prints `check_level=2`, but changing `labels` to
`labels="$(gh api repos/leanprover/lean4/pulls/5343 --jq '.labels')"`
prints `check_level=0`.