Use non-large arm runner

As visible in repo `Insights` `Actions Usage Metrics` the
`arm-4core-linux-ubuntu24.04` variant is a `hosted-larger` which may not
be free.

Use `ubuntu-24.04-arm` instead.

See: https://github.blog/changelog/2025-01-16-linux-arm64-hosted-runners-now-available-for-free-in-public-repositories-public-preview/

.github/workflows/nix.yml

@@ -25,7 +25,7 @@ jobs:
             base: ubuntu-24.04 # always x86_64-linux-gnu
           - os: linux
             cpu: aarch64
-            base: arm-4core-linux-ubuntu24.04 # always aarch64-linux-gnu
+            base: ubuntu-24.04-arm # always aarch64-linux-gnu
         nix:
           - 24.05