(DO NOT MERGE) Give this a full run of x86_64-gnu

Author: Zalathar

src/ci/github-actions/jobs.yml

@@ -97,6 +97,8 @@ pr:
     <<: *job-linux-16c
   - image: x86_64-gnu-tools
     <<: *job-linux-16c
+  - image: x86_64-gnu
+    <<: *job-linux-4c
 
 # Jobs that run when you perform a try build (@bors try)
 # These jobs automatically inherit envs.try, to avoid repeating