ci: drop backticks from check-run summary to satisfy shellcheck SC2016

Backticks inside the single-quoted -f 'output[summary]=...' string
looked like command substitution to shellcheck, tripping actionlint.

Author: ericallam

.github/workflows/changesets-pr.yml

@@ -96,4 +96,4 @@ jobs:
             -f status=completed \
             -f conclusion=success \
             -f 'output[title]=Auto-pass for changeset release PR' \
-            -f 'output[summary]=Required check auto-satisfied for `changeset-release/main` PRs. Full CI ran on the underlying commits before they landed on main.'
+            -f 'output[summary]=Required check auto-satisfied for changeset-release/main PRs. Full CI ran on the underlying commits before they landed on main.'