Enable macOS checks on CI (#1193)

## Summary

Enables tests for macOS in CI, using the M1 runners (which are free in
public, but count against our quota in private
repos). For now, I'm just running them on `main` to save quota.

I did the math, and the M1 runners are the best value:

![Screenshot 2024-01-30 at 9 33
36 PM](https://github.com/astral-sh/puffin/assets/1309177/bd5a14b6-740c-487f-bcad-81c0fce5b62e)

Closes #1053.

.github/workflows/ci.yml

@@ -72,6 +72,33 @@ jobs:
           source .env
           cargo nextest run --all --status-level skip --failure-output immediate-final --no-fail-fast -j 12
 
+  macos:
+    runs-on:
+      labels: macos-14
+    name: "cargo test | macos"
+    steps:
+      - uses: actions/checkout@v4
+      - name: "Install bootstrap dependencies"
+        run: |
+          brew install coreutils
+      - name: "Install required Python versions"
+        run: |
+          scripts/bootstrap/install.sh
+      - name: "Install Rust toolchain"
+        run: rustup show
+      - uses: rui314/setup-mold@v1
+      - name: "Install cargo nextest"
+        uses: taiki-e/install-action@v2
+        with:
+          tool: cargo-nextest
+      - uses: Swatinem/rust-cache@v2
+        with:
+          save-if: ${{ github.ref == 'refs/heads/main' }}
+      - name: "Tests"
+        run: |
+          source .env
+          cargo nextest run --all --status-level skip --failure-output immediate-final --no-fail-fast -j 12
+
   # TODO(konstin): Merge with the cargo-test job once the tests pass
   windows:
     runs-on: windows-latest-large