CI: switch dist-x86_64-musl and i686-gnu to free runners

Author: marcoieni

src/ci/github-actions/jobs.yml

@@ -205,13 +205,13 @@ auto:
   - image: dist-x86_64-musl
     env:
       CODEGEN_BACKENDS: llvm,cranelift
-    <<: *job-linux-8c
+    <<: *job-linux-4c
 
   - image: dist-x86_64-netbsd
     <<: *job-linux-4c
 
   - image: i686-gnu
-    <<: *job-linux-8c
+    <<: *job-linux-4c
 
   - image: i686-gnu-nopt
     <<: *job-linux-8c