ci: Pin the nightly toolchain for aarch64 jobs

Pin aarch64-unknown-linux-gnu and aarch64-apple-darwin to
nightly-2025-02-07 until [1] makes it to a Rust nightly.

[1]: llvm/llvm-project#127804

Author: tgross35

libm/.github/workflows/main.yaml

@@ -22,8 +22,12 @@ jobs:
         include:
         - target: aarch64-apple-darwin
           os: macos-15
+          # FIXME: pinned due to https://github.com/llvm/llvm-project/issues/127804
+          channel: nightly-2025-02-07
         - target: aarch64-unknown-linux-gnu
           os: ubuntu-24.04-arm
+          # FIXME: pinned due to https://github.com/llvm/llvm-project/issues/127804
+          channel: nightly-2025-02-07
         - target: aarch64-pc-windows-msvc
           os: windows-2025
           build_only: 1 # Can't run on x86 hosts