ci: drone: test exec runner

.drone.yml

@@ -0,0 +1,14 @@
+---
+kind: pipeline
+type: exec
+name: exec-runner-test
+
+platform:
+  os: linux
+  arch: amd64
+
+steps:
+- name: test
+  commands:
+  - nix shell 'nixpkgs#hello' -c hello
+...