net/concourse-fly: Update to 6.7.1

patch-vendor_golang.org_x_sys_unix_ztypes__freebsd__arm64.go is removed
because it was integrated upstream.

Thanks to dmgk@, the Makefile is switched to use USES=go:modules and cleaned up
in preparation for the upcoming Go 1.16 release; see PR 251256 [1].

PR: 251256
Submitted by: dmgk@ [1]