Add ORDER BY to JobSetStateIfRunningMany to try and fix intermittency

[brandur]

Here, try to fix a test intermittency problem that I ran into while
trying to fix a different intermittency problem. Here's the error:

--- FAIL: TestDriverRiverDatabaseSQLPgx (0.01s)
driver_test.go:72: Reusing idle postgres schema "river_2025_05_22t23_56_07_schema_07" [user facing: "river_2025_05_22t23_56_07_schema_07"] after cleaning in 12.716292ms [25 generated] [506 reused]
driver_test.go:72: Driver does not support listener; skipping listener tests
--- FAIL: TestDriverRiverDatabaseSQLPgx/JobSetStateIfRunningMany_MultipleJobsAtOnce (0.00s)
    riverdrivertest.go:2663: TestTx using postgres schema: river_2025_05_22t23_56_07_schema_14
    riverdrivertest.go:2679:
        Error Trace:    /Users/brandur/Documents/projects/river/internal/riverinternaltest/riverdrivertest/riverdrivertest.go:2679
        Error:          Not equal:
                expected: "completed"
                actual  : "retryable"

                Diff:
                --- Expected
                +++ Actual
                @@ -1,2 +1,2 @@
                -(rivertype.JobState) (len=9) "completed"
                +(rivertype.JobState) (len=9) "retryable"

        Test:           TestDriverRiverDatabaseSQLPgx/JobSetStateIfRunningMany_MultipleJobsAtOnce
riverdbtest.go:289: Checked in postgres schema "river_2025_05_22t23_56_07_schema_07"; 8 idle schema(s) [30 generated] [1000 reused]
FAIL
FAIL    github.com/riverqueue/river     22.974s
FAIL

It stems from these lines in the driver tests:

jobsAfter, err := exec.JobSetStateIfRunningMany(ctx, setStateManyParams(
    riverdriver.JobSetStateCompleted(job1.ID, now, []byte(`{"a":"b"}`)),
    riverdriver.JobSetStateErrorRetryable(job2.ID, future, makeErrPayload(t, now), nil),
    riverdriver.JobSetStateCancelled(job3.ID, now, makeErrPayload(t, now), nil),
))
require.NoError(t, err)
completedJob := jobsAfter[0]
require.Equal(t, rivertype.JobStateCompleted, completedJob.State)

I think the problem is that when selecting rows to return from
JobSetStateIfRunningMany. Usually these return in input order, but SQL
doesn't guarantee it, so occasionally jobs come back in the wrong order
and we get the retryable job before the completed job.

Here, we add an ORDER BY on the final SELECT. I only add it on one
side of the UNION which if I'm reading this correctly [1], should be
enough.

[1] https://www.postgresql.org/message-id/16814.1280268424%40sss.pgh.pa.us