change the build system type detection to not rely on the refdata signature

The logic to detect the build system type was based on the reference data signature on the job table to match at least one signature on the reference_data_signatures table. This is not always true as reported in https://bugzilla.mozilla.org/show_bug.cgi?id=1033107.
The new logic is inspecting the reference_data_signature table to retrieve a dictionary of repo => build system type that is cached.