build: clamp Cirrus core count to 2.

There's no real added value to making this higher and the ARM box
can barely handle the load.

.cirrus.yml

@@ -17,5 +17,5 @@ task:
        apt-get install -y gcc make
   script:
    - cc --version
-   - ./tools/ci-build.sh
+   - ./tools/ci-build.sh --cores=2
    - make check