vector: build weed-vector without a system protoc (#12317)

* vector: build without a system protoc

lance's build scripts run $PROTOC and fail without one. Point it at the
protoc that protoc-bin-vendored ships, at the version in Cargo.lock, so
building inside seaweed-vector/ needs only the Rust toolchain. An explicit
PROTOC still wins, and a registry without the crate falls back to PATH.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011UpvafvK8JpagF94MHWqwK

* test(s3vectors): build weed-vector inside its workspace

Cargo reads seaweed-vector/.cargo/config.toml only when it runs there, so
build from that directory to pick up the vendored protoc.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011UpvafvK8JpagF94MHWqwK

* ci(vector): drop the PROTOC step the workspace config replaces

Builds run inside seaweed-vector/, where the cargo config already points
lance at the vendored protoc.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011UpvafvK8JpagF94MHWqwK

* test(s3vectors): resolve CARGO_TARGET_DIR before changing directory

A relative CARGO_TARGET_DIR given to make would point cargo, which now runs
in seaweed-vector/, and the cp after it at different directories.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011UpvafvK8JpagF94MHWqwK

* vector: pick the vendored protoc on every Linux host it ships for

protoc-bin-vendored also carries ppc64le, s390x and x86_32 binaries.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011UpvafvK8JpagF94MHWqwK

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

test/s3vectors/Makefile

@@ -7,6 +7,7 @@ endif
 VOLUME := ./weed-volume
 CARGO_PROFILE ?= release
 CARGO_TARGET_DIR ?= $(abspath ../../seaweed-vector/target)
+override CARGO_TARGET_DIR := $(abspath $(CARGO_TARGET_DIR))
 CARGO_OUTPUT := $(CARGO_TARGET_DIR)/$(if $(filter dev,$(CARGO_PROFILE)),debug,$(CARGO_PROFILE))
 export CARGO_TARGET_DIR
 
@@ -18,7 +19,7 @@ $(WEED): FORCE
 	go build -o $(WEED) ../../weed
 
 $(VEC): FORCE
-	cargo build --locked --profile $(CARGO_PROFILE) --manifest-path ../../seaweed-vector/Cargo.toml -p weed-vector-server
+	cd ../../seaweed-vector && cargo build --locked --profile $(CARGO_PROFILE) -p weed-vector-server
 	cp $(CARGO_OUTPUT)/weed-vector $(VEC)
 
 ifneq ($(WORKER),)