fix: inline ARM64 atomics in Sprig image build (#9291)

## Summary

Sprig's ARM64 container fails to link because bundled D-Bus emits a call
to GCC's `__aarch64_ldadd4_sync` helper, which Rust's static musl link
does not supply. AMD64 succeeds, but the failed ARM64 job blocks
publication of the multi-architecture image.

Set the ARM64 musl C compiler flags to `-mno-outline-atomics` in the
image builder. This emits baseline ARM64 atomic instructions inline
instead of requiring the missing helper. Other targets and the runtime
image configuration are unchanged.

### Related issue

No duplicate fix found. [Failing
build](https://github.com/block/buzz/actions/runs/42667527024/job/129335071943).
The [previous main
build](https://github.com/block/buzz/actions/runs/42642040718) has the
same failure, before the MCP dependency update.

### Testing

The exact local ARM64 image build passed: `docker build --platform
linux/arm64 --file Dockerfile.sprig --tag buzz-sprig:arm64-link-fix .`.
The resulting `sprig --version` command passed. The `buzz-agent`
personality reached its expected missing-provider configuration error.
Broad local checks were not run.

Generated with Codex

Signed-off-by: Salman Mohammed <smohammed@squareup.com>

Dockerfile.sprig

@@ -16,6 +16,10 @@ RUN apk add --no-cache \
       protoc
 WORKDIR /build
 COPY . .
+# GCC's outlined ARM64 atomics require libgcc helpers that Rust's static musl
+# link does not provide (notably __aarch64_ldadd4_sync in vendored D-Bus).
+# Emit baseline ARM64 atomics inline; cc-rs applies this only to that target.
+ENV CFLAGS_aarch64_unknown_linux_musl="-mno-outline-atomics"
 RUN cargo build --locked --profile sprig -p sprig \
     && strip target/sprig/sprig