Build the Rust worker against the protoc that ships with the build (#10881)

* worker: compile plugin.proto with the protoc that ships with the build

seaweed-volume already does this: protoc-bin-vendored carries the binary, so
the build needs no package manager and every build sees the same version. An
explicit PROTOC still wins, which is what lets the lance crates - whose own
build scripts read the same variable - share it.

Claude-Session: https://claude.ai/code/session_01Rkp1Mw5E89Jp6dzJFYiMrm

* ci: point the worker builds at the vendored protoc

The jobs installed protobuf-compiler for lance's build scripts. They read
PROTOC, so pointing it at the binary protoc-bin-vendored already puts in the
registry serves them without a system package - one less apt call on the way
to a release, and the same protoc a developer's build uses.

Claude-Session: https://claude.ai/code/session_01Rkp1Mw5E89Jp6dzJFYiMrm

* docs: say what the worker build needs from protoc

The lance crates' build scripts are the ones that need it, not ours, and they
take the same vendored binary.

Claude-Session: https://claude.ai/code/session_01Rkp1Mw5E89Jp6dzJFYiMrm

seaweed-worker/crates/core/build.rs

@@ -1,4 +1,12 @@
 fn main() -> Result<(), Box<dyn std::error::Error>> {
+    // Use the protoc that ships with protoc-bin-vendored rather than a system
+    // one, so the build needs no package manager and always sees the same
+    // version. An explicit PROTOC still wins, for packagers supplying their own
+    // and for the lance crates, whose own build scripts read the same variable.
+    if std::env::var_os("PROTOC").is_none() {
+        std::env::set_var("PROTOC", protoc_bin_vendored::protoc_bin_path()?);
+    }
+
     // Compiled straight out of the Go tree, the way seaweed-volume already reads
     // filer.proto, so the contract cannot drift from a vendored copy.
     tonic_build::configure()