build: require go1.12

[jseldess]

PR: cockroachdb/cockroach#38379

From release notes:

Building CockroachDB from source now requires Go 1.12 or higher. [#38379][#38379] {% comment %}doc{% endcomment %}

[jseldess]

Already documented: http://localhost:4000/docs/dev/install-cockroachdb-mac.html#build-from-source-1