ci: Pin the nightly toolchain for aarch64-unknown-linux-gnu

Pin aarch64-unknown-linux-gnu to nightly-2025-02-07 until [1] is
resolved.

[1]: llvm/llvm-project#127804

Author: tgross35

.github/workflows/main.yml

@@ -20,7 +20,8 @@ jobs:
           rust: nightly
         - target: aarch64-unknown-linux-gnu
           os: ubuntu-latest
-          rust: nightly
+          # FIXME: pinned due to https://github.com/llvm/llvm-project/issues/127804
+          rust: nightly-2025-02-07
         - target: aarch64-pc-windows-msvc
           os: windows-latest
           rust: nightly