switch mingw-1 job to free runners

Author: marcoieni

src/ci/github-actions/jobs.yml

@@ -431,14 +431,15 @@ auto:
       NO_DOWNLOAD_CI_LLVM: 1
     <<: *job-windows-8c
 
+  # x86_64-mingw is split into two jobs to run tests in parallel.
   - image: x86_64-mingw-1
     env:
       SCRIPT: make ci-mingw-x
       RUST_CONFIGURE_ARGS: --build=x86_64-pc-windows-gnu
       # We are intentionally allowing an old toolchain on this builder (and that's
       # incompatible with LLVM downloads today).
       NO_DOWNLOAD_CI_LLVM: 1
-    <<: *job-windows-8c
+    <<: *job-windows
 
   - image: x86_64-mingw-2
     env: