chore: updated .drone.yml

* added separate `pull-image` step to accurately show how much pure
  build takes (without the time it takes to pull a fresh image)

.drone.yml

@@ -6,9 +6,20 @@ platform:
   arch: amd64
 
 steps:
-- name: build
+- name: pull-image
   pull: always
   image: immawanderer/archlinux-cdev:latest
+  clone:
+    disable: true
+  commands:
+  - uname -r
+  - cat /etc/os-release
+
+- name: build
+  pull: if-not-exists
+  image: immawanderer/archlinux-cdev:latest
+  depends_on:
+    - pull-image
   commands:
   - cmake CMakeLists.txt
   - make