implement golangci-lint on mantle code as GH Action

[miabbott]

Feature Request

Implement golangci-lint checks on the mantle code for each PR as part of a GH Action

Desired Feature

Configure a GH Action that runs golangci-lint on each PR, similar to what is done for Butane and Ignitioon

Example Usage

$ git clone https://github.com/coreos-assembler
$ pushd coreos-assembler/mantle
$ golanci-lint run -v 

Other Information

Opening this issue to test the appetite for linting the Go code in mantle. I started down the path as a follow-up to https://github.com/coreos/fedora-coreos-tracker/issues/1096 but thought I would see if this was desirable before investing more time.

[cgwalters]

SGTM, I think it'd be good to not run it as part of make check and stop downloading/building it there by default at least.