Avoid compiling Cython and installing gcc etc. in code style CI job since if's not used there.

.github/workflows/ci.yml

@@ -38,7 +38,7 @@ jobs:
           - os: ubuntu-18.04
             python-version: 3.7
             backend: "c,cpp"
-            env: { TEST_CODE_STYLE: 1 }
+            env: { TEST_CODE_STYLE: 1, NO_CYTHON_COMPILE: 1 }
             extra_hash: "-codestyle"
           # Limited API
           - os: ubuntu-18.04

Tools/ci-run.sh

 #!/usr/bin/bash
 
 # Set up compilers
-if [ "${OS_NAME##ubuntu*}" == "" ]; then
+if [ "${OS_NAME##ubuntu*}" == "" - "$TEST_CODE_STYLE" != "1" ]; then
   echo "Installing requirements [apt]"
   sudo apt-add-repository -y "ppa:ubuntu-toolchain-r/test"
   sudo apt update -y -q