Merge #284: ci: Avoid separate pull to work around snap issue

577ccb0 ci: Avoid separate pull to work around snap issue (MarcoFalke)

Pull request description:

  Work around canonical/snapcraft#5316

  https://cirrus-ci.com/task/6347928203165696?logs=ci#L0:

  ```
  ...
  snapcraft --use-lxd
  Launching a container.
  Waiting for container to be ready
  Waiting for network to be ready...
  error: cannot perform the following tasks:
  - Run configure hook of "core" snap (run hook "configure": cannot set "core.experimental.snapd-snap": unsupported system option)
  An error occurred when trying to execute 'snap set system experimental.snapd-snap=true' with 'LXD': returned exit code 1.
  Exit status: 2

ACKs for top commit:
  hebasto:
    ACK 577ccb0, I have reviewed the code and it looks OK.

Tree-SHA512: fcbeb0089fdb95b9d2e32e45fc886200caf71d5883165ac6c39d63a40e0399ddc284fab4fbdf83a54ff5704c4ef927b1bc9dde807605ee146856a55da7f84d64

Author: fanquake

.cirrus.yml

@@ -10,7 +10,5 @@ task:
     - snap install snapcraft --classic
     - snap install lxd
     - /snap/bin/lxd init --auto
-  snapcraft_pull_script:
-    - snapcraft pull --use-lxd
-  ci_script:
+  snapcraft_build_script:
     - snapcraft --use-lxd