Move weather field to database.

* src/sql/upgrade-4.sql: New file.
* Makefile.am (dist_sql_DATA): Add it.
* src/schema.sql (Builds)[weather]: New field.
(Builds_weather_evaluation): New index.
* src/cuirass/database.scm (db-get-last-status): New procedure.
(db-update-build-status!): Use it and update the new weather field.
(db-get-builds): Adapt it.
* tests/http.scm (build-query-result): Ditto.