ci: Reduce Alpine musl task to md runner size

Per bitcoin/bitcoin#33480 (comment)

Author: MarcoFalke

.github/workflows/ci.yml

@@ -478,7 +478,7 @@ jobs:
             file-env: './ci/test/00_setup_env_native_previous_releases.sh'
 
           - name: 'Alpine (musl), depends, gui'
-            cirrus-runner: 'ghcr.io/cirruslabs/ubuntu-runner-amd64:24.04-lg'
+            cirrus-runner: 'ghcr.io/cirruslabs/ubuntu-runner-amd64:24.04-md'
             fallback-runner: 'ubuntu-24.04'
             timeout-minutes: 120
             file-env: './ci/test/00_setup_env_native_alpine_musl.sh'