fix: limit the scope of the template average build time query to the last 100 (#19648)

This PR should resolve https://github.com/coder/internal/issues/719 by
limiting the `workspace_builds` rows selected by the query to the most
recent 100 builds of a template, as opposed to all builds in the last
30d. For our own internal templates with the most builds (1700-2000 in a
30d period) this should cut the query execution time by about 80%.

Unless we have some restriction on keeping the 30d period, contract
related or otherwise, this seems like a safe change to make. In addition
to the execution speed improvements it also means the memory for the
query is bounded as well.

If we want to keep a 30d time period for the avg build time value I
think it's worth exploring a purpose built solution such as histogram
structures where the build times could be bucketized by template ID as
they're observed.

---------

Signed-off-by: Callum Styan <callumstyan@gmail.com>

coderd/database/queries/templates.sql

@@ -203,11 +203,11 @@ JOIN provisioner_jobs pj ON
 WHERE
 	template_versions.template_id = @template_id AND
 		(pj.completed_at IS NOT NULL) AND (pj.started_at IS NOT NULL) AND
-		(pj.started_at > @start_time) AND
 		(pj.canceled_at IS NULL) AND
 		((pj.error IS NULL) OR (pj.error = ''))
 ORDER BY
 	workspace_builds.created_at DESC
+LIMIT 100
 )
 SELECT
 	-- Postgres offers no clear way to DRY this short of a function or other