Makefile: fix go vet/fmt
Makefile: fix go vet/fmt

I have noticed that `go vet` from golang 1.13 ignores the vendor/
subdir, downloading all the modules when invoked in Travis CI env.

As the other go commands, in 1.13 it needs explicit -mod=vendor
flag, so let's provide one.

PS once golang 1.13 is unsupported, we will drop it.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

dqminh/runc,mikebrow/runc,kolyshkin/runc,mikebrow/runc,opencontainers/runc,estesp/runc,rhatdan/runc,estesp/runc,dqminh/runc,opencontainers/runc,dqminh/runc,estesp/runc,xiaochenshen/runc,xiaochenshen/runc,xiaochenshen/runc,rhatdan/runc,opencontainers/runc,kolyshkin/runc,rhatdan/runc,dqminh/runc,rhatdan/runc,opencontainers/runc,kolyshkin/runc,mikebrow/runc,kolyshkin/runc