ci: Restrict mise to 2 concurrent installs (#2984)

Author: sl0thentr0py

.github/workflows/update_lockfiles.yml

@@ -34,6 +34,8 @@ jobs:
       # prebuilt binaries where available); bin/relock then resolves each cell
       # against its matching Ruby.
       - uses: jdx/mise-action@e6a8b3978addb5a52f2b4cd9d91eafa7f0ab959d # v4.2.0
+        env:
+          MISE_JOBS: "2"
         with:
           install: true
           cache: true