place CI fix into common.sh

.drone.yml

@@ -9,8 +9,6 @@ steps:
   # unfortunately we have to do everything in one step because there's no
   # way to share system packages between steps
   commands:
-    # TODO: fix Drone
-    - chmod 1777 /tmp
     # install dependencies
     - apt update && apt install -y libkrb5-dev libpq-dev python3-dev
     - python3 -m venv venv

.drone/common.sh

@@ -1,3 +1,6 @@
+# TODO: fix Drone
+chmod 1777 /tmp
+
 # don't resolve container names to *real* CSC machines
 sed -E '/^(domain|search)[[:space:]]+csclub.uwaterloo.ca/d' /etc/resolv.conf > /tmp/resolv.conf
 cp /tmp/resolv.conf /etc/resolv.conf