GH Actions: speed up slow builds

GitHub has made Linux arm64 runners generally available and running tasks on these images instead of the traditional images can deliver up to a 40% performance boost.

As the fast majority of end-users won't be using Arm yet, I'm not (yet) switching all Linux builds to Arm runners, but for now, I think it makes sense to try and speed up the build by using these images on the slowest builds.

Refs:
* https://github.blog/news-insights/product-news/arm64-on-github-actions-powering-faster-more-efficient-build-systems/
* https://docs.github.com/en/actions/using-github-hosted-runners/using-github-hosted-runners/about-github-hosted-runners#supported-runners-and-hardware-resources

.github/workflows/quicktest.yml

@@ -32,7 +32,7 @@ jobs:
 
         include:
           - php: '5.4'
-            os: 'ubuntu-latest'
+            os: 'ubuntu-24.04-arm'
             custom_ini: false
           # Installing on Windows with PHP 5.4 runs into all sorts of problems with Composer.
           # See this issue for more context (yes, I've seen this problem before):

.github/workflows/test.yml

@@ -189,7 +189,7 @@ jobs:
 
         include:
           - php: '5.4'
-            os: 'ubuntu-latest'
+            os: 'ubuntu-24.04-arm'
             custom_ini: false
           # Installing on Windows with PHP 5.4 runs into all sorts of problems with Composer.
           # Considering PHP 5.4 is ancient, I deem it acceptable to run coverage on Windows on PHP 5.5.