Auto merge of rust-lang#131362 - Kobzol:ci-free-runners-linux-4c, r=<try>

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).

try-job: dist-various-1

Author: bors

.github/workflows/ci.yml

@@ -104,6 +104,9 @@ jobs:
         with:
           fetch-depth: 2
 
+      - name: Free Disk Space (Ubuntu)
+        uses: jlumbroso/free-disk-space@main
+
       # Rust Log Analyzer can't currently detect the PR number of a GitHub
       # Actions build on its own, so a hint in the log message is needed to
       # point it in the right direction.

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