build-style/go: don't set GOBIN

Go 1.5 doesn't permit our use of GOBIN anymore, breaking
cross-compilation (for details, see
https://github.com/golang/go/issues/9769)

In retrospect, I'm not sure why we set GOBIN in the first place;
GOPATH/bin should suffice.

Closes #2337

common/environment/build-style/go.sh

@@ -12,4 +12,3 @@ case "$XBPS_TARGET_MACHINE" in
 esac
 
 export GOPATH="${wrksrc}/_build-${pkgname}-xbps"
-export GOBIN="${wrksrc}/_build-${pkgname}-xbps-bin"