[Rust] Fall back to system protoc

[notpeter]

What/Why: On Illumos current repo will fail at build time because rust-protoc-bin-vendored cannot find a protoc binary. After this change, build will succeed if there's a suitable protoc in your path or a PROTOC environment variable set with a suitable path. This makes it possible to compile zerobus-sdk on Illumos.

Changes:

• Enable zerobus-sdk compilation on platforms without pre-build Google protoc binaries.
• New Behavior: if PROTOC env-var set with path, prefer that protoc over fetching a pre-built binary from Google.

Upstream:

• See: option of not using protocolbuffers/protobuf binary releases? stepancheg/rust-protoc-bin-vendored#9

How is this tested?

I have tested this change building locally on OmniOS as well as running the full test suite under CI on my fork.
Passing run here.

If you would like to add Illumos to your test suite (via vmactions/omnios-vm, happy to add that here too.
Similarly vmactions/freebsd-vm also exists.