Golang ports: use ${WRKDIR} for build artifacts

[lytboris_gmail.com]

Summary

Golang uses $TMPDIR for it's build artifacts by default now. This could lead into issues when /tmp is RAM-backed or no-exec.

So it's better to use ${WRKDIR} as working directory, the same way most of other software does.

Test Plan

Configure tmpfs-based /tmp with 512M quota and try to build net-mgmt/telegraf with or without patch proposed

[dmgk]

LGTM, thanks. I'll commit this after I'm done with updating Go ports.