Currently, a full store path has to be provided to search in
builds. This patch permits to search jobs with a output path or
derivation hash.

Usecase: we are building Docker images with Hydra. The tag of the
Docker image is the hash of the image output path. This patch would
allow us to find back the build job from the tag of a running
container image.

The search query uses the LIKE operator which requires a sequential
scan (it can't use the already existing B-tree index). This new
index (trigram) avoids a sequential scan of the builds table when the
LIKE operator is used.

Here is the analyze of a request on the builds table with this index:

     explain analyze select * from builds where drvpath like '%k3r71gz0gv16ld8rhcp2bb8gb5w1xc4b%';
                                                                 QUERY PLAN
     -----------------------------------------------------------------------------------------------------------------------------------
      Bitmap Heap Scan on builds  (cost=128.00..132.01 rows=1 width=492) (actual time=0.070..0.077 rows=1 loops=1)
        Recheck Cond: (drvpath ~~ '%k3r71gz0gv16ld8rhcp2bb8gb5w1xc4b%'::text)
        ->  Bitmap Index Scan on indextrgmbuildsondrvpath  (cost=0.00..128.00 rows=1 width=0) (actual time=0.047..0.047 rows=3 loops=1)
              Index Cond: (drvpath ~~ '%k3r71gz0gv16ld8rhcp2bb8gb5w1xc4b%'::text)
      Total runtime: 0.206 ms
     (5 rows)

Allow to search builds by hash