This PR does two things, to facilitate ssi being built within a nix flake:
use a local cache directory within $OUT_DIR for the protofetch cache, instead of a global cache
expose a PROTOFETCH_OVERRIDE_REPOSITORY env var, which will instruct the internal protofetch process to fetch the protobuf from a local file instead of trying to directly fetch it from github
This PR does two things, to facilitate
ssibeing built within a nix flake:PROTOFETCH_OVERRIDE_REPOSITORYenv var, which will instruct the internal protofetch process to fetch the protobuf from a local file instead of trying to directly fetch it from github