FR: check gofmt in workflow / run gofmt on merge

[jmr]

Based on https://github.com/golang/geo/pull/135/files#diff-c6d08efc7b53c86c30dc7c658277042da01aab93a74f424baee5fa29edb9fb82R111 it doesn't look like we have checks for properly gofmted code.

[rsned]

#140 added these checks