And if I substitute "random" stuff I get

ERROR: new row for relation "builds" violates check constraint "builds_check"

Without setting much stuff it's like

hydra=# BEGIN;
BEGIN
hydra=*# EXPLAIN ANALYZE  UPDATE builds SET finished = 1 where id=123456;
                                                          QUERY PLAN                                                          
------------------------------------------------------------------------------------------------------------------------------
 Update on builds  (cost=0.57..2.79 rows=0 width=0) (actual time=4.086..4.087 rows=0.00 loops=1)
   Buffers: shared hit=79 read=18 dirtied=10
   ->  Index Scan using builds_pkey on builds  (cost=0.57..2.79 rows=1 width=10) (actual time=0.293..0.294 rows=1.00 loops=1)
         Index Cond: (id = 123456)
         Index Searches: 1
         Buffers: shared hit=3 read=2
 Planning:
   Buffers: shared hit=294
 Planning Time: 1.508 ms
 Trigger nrbuildsfinished: time=0.648 calls=1
 Execution Time: 4.811 ms
(11 rows)

hydra=*# ROLLBACK;
ROLLBACK