fix(build): build vendored opus so arm64 links on hosts with an Intel Homebrew

[yonatangross]

Problem

just desktop-release-build fails at link time on an Apple Silicon Mac that also has an Intel Homebrew installed:

Undefined symbols for architecture arm64:
  "_opus_encoder_create", referenced from:
      opus::Encoder::new::... in libopus-....rlib
  ...
ld: symbol(s) not found for architecture arm64

audiopus_sys's build.rs probes pkg-config first and returns early on a hit, so the vendored opus that the existing CMAKE_POLICY_VERSION_MINIMUM = "3.5" entry exists to compile never gets built. pkg-config's default search path includes /usr/local/lib/pkgconfig — the Intel Homebrew prefix — so an x86_64 libopus.a is statically linked into an arm64 binary.

CI never hits this (no Homebrew opus on the runners), which is why the opus source has to be pinned in-repo rather than left to ambient host state.

Fix

Two env vars in .cargo/config.toml, deliberately both:

var	why
LIBOPUS_NO_PKG	audiopus_sys's own documented bypass. States the intent — but its build.rs never emits cargo:rerun-if-env-changed=LIBOPUS_NO_PKG, so on a tree that already resolved to the system libopus, cargo reuses the cached build script and this reads as inert. (cargo clean -p audiopus_sys does not reliably evict the build-script output dir either.)
OPUS_NO_PKG_CONFIG	pkg-config-rs's per-library opt-out. It is declared via rerun-if-env-changed, so it actually invalidates the cached build script and forces the vendored cmake path to run.

Setting only the first one looks correct and changes nothing, which is why both are here with the reasoning inline.

Verification

Before — build script resolved to the Intel prefix:

cargo:info=Found `Opus` via `pkg_config`.
cargo:rustc-link-search=native=/usr/local/Cellar/opus/1.6.1/lib
cargo:rustc-link-lib=static=opus

After:

cargo:info=Bypassed `pkg-config`.
cargo:info=Building Opus via CMake.
cargo:rustc-link-search=native=.../release/build/audiopus_sys-.../out/lib

just desktop-release-build aarch64-apple-darwin then produces a bundle whose Contents/MacOS/* are 7/7 arm64, and opus is statically linked (no dynamic reference).

Anyone can reproduce the original failure with brew install opus under an Intel Homebrew on an Apple Silicon host.

Scope

One file, config only. No behaviour change on any host that does not have a system libopus — the vendored path is what CI already takes.