build: don't override gopath by default, demote old gopath on override

Builds already vendor through cmd/ so there's no reason to set the GOPATH; it
was also breaking gofail builds. For builds that need to override GOPATH, also
include the old GOPATH as a fallback for dependencies outside cmd/vendor/.

test

@@ -12,6 +12,9 @@ set -e
 
 source ./build
 
+# build tests with vendored dependencies
+etcd_setup_gopath
+
 if [ -z "$PASSES" ]; then
 	PASSES="fmt dep compile build unit"
 fi