GOPATH and GOBIN must be unset before build

[oholiab]

Not entirely sure about GOPATH, but definitely GOBIN needs to be unset if they are set in the user's environment before calling gdp buildpackage

I attempted to build oz with both set (full incantation was DIST=stretch gbp buildpackage -us -uc --git-debian-branch=debian-test) and it failed here:

...
github.com/subgraph/oz/vendor/github.com/twtiger/gosecco/emulator
github.com/subgraph/oz/vendor/github.com/twtiger/gosecco/tester
go install github.com/subgraph/oz/vendor/github.com/twtiger/gosecco/tester: open /home/user/golang/bin/tester: no such file or directory
dh_auto_build: go install -v -p 1 -buildmode=pie -pkgdir=/build/oz-daemon-0.2.6/tmp.VPEoyMctLR github.com/subgraph/oz github.com/subgraph/oz/cmd/oz-daemon github.com/subgraph/oz/cmd/oz-init github.com/subgraph/oz/cmd/oz-mount github.com/subgraph/oz/cmd/oz-ovpn-route-down github.com/subgraph/oz/cmd/oz-ovpn-route-up github.com/subgraph/oz/cmd/oz-seccomp github.com/subgraph/oz/cmd/oz-seccomp-tracer github.com/subgraph/oz/cmd/oz-setup github.com/subgraph/oz/cmd/oz-umount github.com/subgraph/oz/fs github.com/subgraph/oz/ipc github.com/subgraph/oz/network github.com/subgraph/oz/ns github.com/subgraph/oz/openvpn github.com/subgraph/oz/oz github.com/subgraph/oz/oz-daemongithub.com/subgraph/oz/oz-init github.com/subgraph/oz/oz-mount github.com/subgraph/oz/oz-seccomp github.com/subgraph/oz/vendor/github.com/BurntSushi/xdg github.com/subgraph/oz/vendor/github.com/codegangsta/cli github.com/subgraph/oz/vendor/github.com/docker/libcontainer/netlink github.com/subgraph/oz/vendor/github.com/docker/libcontainer/system github.com/subgraph/oz/vendor/github.com/gotk3/gotk3/cairo github.com/subgraph/oz/vendor/github.com/gotk3/gotk3/gdk github.com/subgraph/oz/vendor/github.com/gotk3/gotk3/glib github.com/subgraph/oz/vendor/github.com/gotk3/gotk3/gtk github.com/subgraph/oz/vendor/github.com/gotk3/gotk3/pango github.com/subgraph/oz/vendor/github.com/j-keck/arping github.com/subgraph/oz/vendor/github.com/kr/pty github.com/subgraph/oz/vendor/github.com/milosgajdos83/tenus github.com/subgraph/oz/vendor/github.com/naegelejd/go-acl github.com/subgraph/oz/vendor/github.com/op/go-logging github.com/subgraph/oz/vendor/github.com/subgraph/constants github.com/subgraph/oz/vendor/github.com/subgraph/go-xdgdirs github.com/subgraph/oz/vendor/github.com/twtiger/gosecco github.com/subgraph/oz/vendor/github.com/twtiger/gosecco/asm github.com/subgraph/oz/vendor/github.com/twtiger/gosecco/checker github.com/subgraph/oz/vendor/github.com/twtiger/gosecco/compiler github.com/subgraph/oz/vendor/github.com/twtiger/gosecco/constants github.com/subgraph/oz/vendor/github.com/twtiger/gosecco/data github.com/subgraph/oz/vendor/github.com/twtiger/gosecco/emulator github.com/subgraph/oz/vendor/github.com/twtiger/gosecco/native github.com/subgraph/oz/vendor/github.com/twtiger/gosecco/parser github.com/subgraph/oz/vendor/github.com/twtiger/gosecco/precompilation github.com/subgraph/oz/vendor/github.com/twtiger/gosecco/simplifier github.com/subgraph/oz/vendor/github.com/twtiger/gosecco/tester github.com/subgraph/oz/vendor/github.com/twtiger/gosecco/tree github.com/subgraph/oz/vendor/github.com/twtiger/gosecco/unifier github.com/subgraph/oz/vendor/golang.org/x/sys/unix github.com/subgraph/oz/xpra returned exit code 1
debian/rules:13: recipe for target 'override_dh_auto_build' failed
make[1]: *** [override_dh_auto_build] Error 1
make[1]: Leaving directory '/build/oz-daemon-0.2.6'
debian/rules:10: recipe for target 'build' failed
make: *** [build] Error 2
dpkg-buildpackage: error: debian/rules build gave error exit status 2
I: copying local configuration
E: Failed autobuilding of package
I: unmounting dev/ptmx filesystem
I: unmounting dev/pts filesystem
I: unmounting dev/shm filesystem
I: unmounting proc filesystem
I: unmounting sys filesystem
I: Cleaning COW directory
I: forking: rm -rf /var/cache/pbuilder/build/cow.25049
gbp:error: 'git-pbuilder -us -uc' failed: it exited with 1
user@subgraph/pts/2:~/oz [debian-test]>

As you can see, my $GOBIN from my .zshrc set to $HOME/golang/bin has polluted the build environment and caused a failure. This is still the case with $GOPATH unset, and if I unset both $GOPATH and $GOBIN it works just fine.

Seems like both of these should be unset at the lowest common entry point for all build scripts.