Recent CI flakiness with DB pool acquire timeouts

[bgentry]

I've seen a bunch of these in the last few days: https://github.com/riverqueue/river/actions/runs/13346849278/job/37278355065?pr=760

--- FAIL: TestDriverRiverPgxV5 (33.34s)
    --- FAIL: TestDriverRiverPgxV5/GetMigrationLines ([10](https://github.com/riverqueue/river/actions/runs/13346849278/job/37278355065?pr=760#step:7:11).00s)
        riverdrivertest.go:55: Failed to acquire pool for test DB: context deadline exceeded
FAIL

At a glance I don't see how this could have been caused by any specific recent changes but it's probably in part a sign that we need to refocus on speeding up the test suite soon.