ci: Reuse the Go cache that main saves in the regression-report workflow (#7119)

Follow-up to #7105 (added the
regression-report workflow) and
#7108 (authenticated the latest
release lookup).

The workflow used its own cache key, which is only written by runs on
main. It never runs on main, so every run started with a cold Go cache.
Restore the key that the Linux test job saves instead. A full run goes
from 505-587s to 292s, a run with no acceptance changes from 370s to
196s.

This pull request and its description were written by Isaac.

---------

Co-authored-by: Isaac <no-reply@databricks.com>

Author: denik

.github/workflows/regression-report.yml

@@ -34,7 +34,9 @@ jobs:
       - name: Setup build environment
         uses: ./.github/actions/setup-build-environment
         with:
-          cache-key: regression-report
+          # The Go cache is only saved by runs on main, and this workflow never runs there,
+          # so it reuses the key push.yml saves for its Linux test job.
+          cache-key: test-direct
 
       # `git diff --merge-base origin/main` needs origin/main and enough history. This must run
       # AFTER setup-build-environment, whose own checkout would re-shallow the repo.