Fix building Web Viewer on macOS (#10828)

### Related
Closes #9849

### What
Basically mac has a fun combination where you can have `clang`/`llvm`
components but not `llvm-ar`. Mac's `ar` pads bytes in way that make
llvm barf. Our pixi env for c++ has the `cxx-compiler` package which has
a more full fledged llvm install. On mac on activation set `AR` to point
to `llvm-ar`. It works on my machine! I'm not sure if we should have a
second person try or if we should setup a test for the web build on mac
just for this weird edge case.

Author: ntjohnson1

pixi.toml

@@ -53,6 +53,13 @@ EXECUTABLE_EXTENSION = ""
 # The executable extension for binaries on the current platform.
 EXECUTABLE_EXTENSION = ".exe"
 
+# TODO(rust-lang/stacker#127): Work around until wasm build on mac is resolved
+[target.osx-64.activation.env]
+AR = "${PIXI_PROJECT_ROOT}/.pixi/envs/cpp/bin/llvm-ar"
+
+[target.osx-arm64.activation.env]
+AR = "${PIXI_PROJECT_ROOT}/.pixi/envs/cpp/bin/llvm-ar"
+
 # python-dev
 
 [feature.python-dev.activation.env]