make gen failed: file does not exist

[sjtuzbk]

After I cloned the code locally, I executed the make gen command, but got the following error.

➜  api git:(master) ✗ make gen  
./clean.sh
./gen.sh
authentication/v1alpha1/policy.proto:23:8:import "google/api/field_behavior.proto": file does not exist
make[1]: *** [Makefile.core.mk:34: gen-proto] Error 100
make: *** [gen] Error 2

[sschepens]

this is definitely affecting builds on macOS, running on a Linux VM doesn't fail though