Downsize builder for mingw-check

The infra-team is continuously monitoring the efficiency of the build
system in an effort to improve overall build times and resource usage.
The builder for the `mingw-check` target have used much less resources
than allocated in the past, so we are testing a smaller builder size for
it.

.github/workflows/ci.yml

@@ -266,7 +266,7 @@ jobs:
             os: ubuntu-20.04-8core-32gb
             env: {}
           - name: mingw-check
-            os: ubuntu-20.04-8core-32gb
+            os: ubuntu-20.04-4core-16gb
             env: {}
           - name: test-various
             os: ubuntu-20.04-8core-32gb

src/ci/github-actions/ci.yml

@@ -73,6 +73,10 @@ x--expand-yaml-anchors--remove:
   - &base-job
     env: {}
 
+  - &job-linux-4c
+    os: ubuntu-20.04-4core-16gb
+    <<: *base-job
+
   - &job-linux-8c
     os: ubuntu-20.04-8core-32gb
     <<: *base-job
@@ -422,7 +426,7 @@ jobs:
             <<: *job-linux-8c
 
           - name: mingw-check
-            <<: *job-linux-8c
+            <<: *job-linux-4c
 
           - name: test-various
             <<: *job-linux-8c