package/mender-artifact: do not download modules during the build process

package/mender-artifact: do not download modules during the build process

By default, go will attempt to download needed modules before building, which
is not desirable. This behavior also causes permission issues when cleaning,
as go downloads modules as read-only by default. Because mender-artifact
includes the modules in the vendor directory, mod=vendor prevents the package
from downloading the go modules during the build process and prevents
permission issues when cleaning.

Fixes:
http://autobuild.buildroot.net/results/d5bcaca73ae74fe8b0ebd39b6331564cd639fb66

Signed-off-by: Mirza Krak <mirza.krak@northern.tech>
Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>