Auto merge of #131362 - Kobzol:ci-free-runners-linux-4c, r=Mark-Simulacrum

CI: use free runners for 4-core Linux jobs

It looks like the [free runners](https://docs.github.com/en/actions/using-github-hosted-runners/using-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories) already have the same spec as the `4c` custom "large" runner (4 cores, 16 GiB of memory, Ubuntu 20.04).

Author: bors

src/ci/github-actions/jobs.yml

@@ -5,7 +5,7 @@ runners:
     env: { }
 
   - &job-linux-4c
-    os: ubuntu-20.04-4core-16gb
+    os: ubuntu-20.04
     <<: *base-job
 
   - &job-linux-8c