Switch to GitHub Actions public ubuntu-24.04-arm runner (#447)

In January 2025, GitHub Actions announced preview support for public
ARM GitHub Actions runners (previously only available when using the
custom runners group feature):
https://github.blog/changelog/2025-01-16-linux-arm64-hosted-runners-now-available-for-free-in-public-repositories-public-preview/

The public runners were then GAed this month:
https://github.blog/changelog/2025-08-07-arm64-hosted-runners-for-public-repositories-are-now-generally-available/

The Python CNB has been successfully using the public runners since
then (in the first few weeks there was an issue with crashes due to
the public runners using newer CPU model, but that's since been
resolved).

The public runners are faster to boot than the custom runners, since
GitHub has a slack pool of them. (When comparing test end to end times,
bear in mind that GitHub doesn't include the queue time in the reported
job time.)

As such, for any use-case that doesn't need a larger number of CPUs,
we should use the public runners instead. (Even for CPU bound workloads,
the slower boot time often outweighs the benefit of having more CPUs.)

GUS-W-19324156.

Author: edmorley

.github/workflows/ci.yml

@@ -60,7 +60,7 @@ jobs:
         run: cargo test --locked -- --ignored
 
   print-pack-getting-started-output:
-    runs-on: ${{ matrix.target == 'aarch64-unknown-linux-musl' && 'pub-hk-ubuntu-24.04-arm-medium' || 'ubuntu-24.04' }}
+    runs-on: ${{ matrix.target == 'aarch64-unknown-linux-musl' && 'ubuntu-24.04-arm' || 'ubuntu-24.04' }}
     strategy:
       matrix:
         target: ["aarch64-unknown-linux-musl", "x86_64-unknown-linux-musl"]