sql: potential flake in TestLogic/5node-spec-planning/distsql_union/Except

[mgartner]

Seen in this build:

------- Stdout: -------
=== RUN   TestLogic/5node-spec-planning/distsql_union/Except
    logic.go:2704:
        testdata/logic_test/distsql_union:348: (SELECT x FROM xyz WHERE x < 2) EXCEPT (SELECT x FROM xyz WHERE x >= 2) ORDER BY x
        expected success, but found
        (58C01) inbox communication error: rpc error: code = Canceled desc = context canceled
        inbox.go:361: in Next()
    logic.go:2430:
         pq: inbox communication error: rpc error: code = Canceled desc = context canceled
    logic.go:1976:
         testdata/logic_test/distsql_union:353: too many errors encountered, skipping the rest of the input
--- done: testdata/logic_test/distsql_union with config 5node-spec-planning: 38 tests, 3 failures
--- done: testdata/logic_test/distsql_numtables with config 5node-spec-planning: 27 tests, 0 failures
--- done: testdata/logic_test/distsql_crdb_internal with config 5node-spec-planning: 8 tests, 0 failures
--- progress: testdata/logic_test/distsql_agg: 35 statements/queries
--- progress: testdata/logic_test/hash_join_dist: 16 statements/queries
--- done: testdata/logic_test/hash_join_dist with config 5node-spec-planning: 18 tests, 0 failures
--- total progress: 159492 statements/queries
--- progress: testdata/logic_test/distsql_distinct_on: 8 statements/queries
--- done: testdata/logic_test/distsql_distinct_on with config 5node-spec-planning: 15 tests, 0 failures
--- progress: testdata/logic_test/distsql_agg: 81 statements/queries
--- done: testdata/logic_test/distsql_agg with config 5node-spec-planning: 81 tests, 0 failures
            --- FAIL: TestLogic/5node-spec-planning/distsql_union/Except (0.13s)