Fix build for FreeBSD

Matthias Fechnerrequested to merge
mfechner/glaz:mfechner-e1100ed8-patch-7538 into main

There is no precompiled vendored protoc available. Error: Error { os: "freebsd", arch: "x86_64" } warning: build failed, waiting for other jobs to finish...

Allow the system to define an environment variable PROTOC to point to an already existing one.

This modification should not have any impact on the Linux or MacOS system if a vendorered binary already exists.

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