CI: Switch macos aarch64 to nightly

We will be promoting aarch64-apple-darwin to tier 1 soon via
rust-lang/rust#128592. This updates our CI
so that aarch64-apple-darwin runs the full test suite on nightly to
keep this in-line with tier-1 support.

.github/workflows/main.yml

@@ -148,9 +148,9 @@ jobs:
           os: macos-13
           rust: nightly
           other: x86_64-apple-ios
-        - name: macOS aarch64 stable
+        - name: macOS aarch64 nightly
           os: macos-14
-          rust: stable
+          rust: nightly
           other: x86_64-apple-darwin
         - name: Windows x86_64 MSVC stable
           os: windows-latest