software/gitlab: unset $GOBIN

gitaly's Makefile forcibly sets $GOPATH to install in _build
folder. Once gitaly is built, it is expected to be in _build/bin/gitaly
and copied from there.

With 8eac67a5 (golang: Prepare for future GOPATH removal, 2021-02-26) the
env.sh sets GOBIN, to its own $GOPATH/bin

Since gitaly's build does not set $GOBIN, even if it overrides $GOPATH, the
final bin/gitaly is not in $GOPATH/bin/gitaly (with $GOPATH from Makefile),
but in $GOBIN/gitaly (with $GOBIN from env.sh)

To prevent this, unset $GOBIN before running make, which keeps the old
behavior of only using the $GOPATH from the Makefile