Add build_info fallback to latency queries in Explorer dashboard

[brettimus]

Fixes #77

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Use

* on(instance, job) group_left(version, commit) (last_over_time(build_info[$__rate_interval]) or on (instance, job) up)

instead of

* on(instance, job) group_left(version, commit) last_over_time(build_info[$__rate_interval])

in the latency histogram queries.

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Tested importing dashboard locally, and it works: