sql: `TestLogic_distinct_on` is flaky under the `fakedist-disk` config.

[ZhouXing19]

Can reproduce it on the current master branch sha 0dc5b09fdf0

> ./dev test pkg/sql/logictest/tests/fakedist-disk -f=TestLogic_distinct_on --stress 


==================== Test output for //pkg/sql/logictest/tests/fakedist-disk:fakedist-disk_test (run 18 of 1000):
metamorphic: use COCKROACH_RANDOM_SEED=-1664273716249860662 for reproduction
...
.../pkg/sql/logictest/testdata/logic_test/distinct_on:403: SELECT * FROM (SELECT DISTINCT ON (x) x, y FROM xyz ORDER BY x, y) WHERE y = 1
expected:
    1  1
    4  1
  
but found (query options: "rowsort" -> ignore the following ordering of rows) :
    4  1

Tried with the failed RANDOM_SEED, doesn't seem to be able to reproduce the error deterministically.

> ./dev test pkg/sql/logictest/tests/fakedist-disk -f=TestLogic_distinct_on --ignore-cache -- --test_env=COCKROACH_RANDOM_SEED=-1664273716249860662

//pkg/sql/logictest/tests/fakedist-disk:fakedist-disk_test               PASSED in 3.1s