flake: TestMultipleLifecycleExecutors

[ethanndickson]

Spotted on nightly-gauntlet, macOS + Postgres:
https://github.com/coder/coder/actions/runs/13668233669

     t.go:109: 2025-03-05 04:06:51.133 [erro]  coderd.autobuild: failed to transition workspace  workspace_id=28def2c4-c0f2-4443-89e2-a39723c41474  workspace_name=elated-pike1-pBI ...
        error= transition workspace:
                   github.com/coder/coder/v2/coderd/autobuild.(*Executor).runOnce.func2.1
                       /Users/runner/work/coder/coder/coderd/autobuild/lifecycle_executor.go:367
                 - execute transaction:
                   github.com/coder/coder/v2/coderd/database.(*sqlQuerier).runTx
                       /Users/runner/work/coder/coder/coderd/database/db.go:213
                 - build workspace with transition "start":
                   github.com/coder/coder/v2/coderd/autobuild.(*Executor).runOnce.func2.1.1
                       /Users/runner/work/coder/coder/coderd/autobuild/lifecycle_executor.go:277
                 - build tx:
                   github.com/coder/coder/v2/coderd/wsbuilder.(*Builder).Build
                       /Users/runner/work/coder/coder/coderd/wsbuilder/wsbuilder.go:240
                 - execute transaction:
                   github.com/coder/coder/v2/coderd/database.(*sqlQuerier).runTx
                       /Users/runner/work/coder/coder/coderd/database/db.go:193
                 - execute transaction:
                   github.com/coder/coder/v2/coderd/database.(*sqlQuerier).runTx
                       /Users/runner/work/coder/coder/coderd/database/db.go:193
                 - pq: duplicate key value violates unique constraint "workspace_builds_workspace_id_build_number_key"

Looks like this constraint can be reasonably tripped by concurrent builds?