CI: mypy: make compatible with "pmbootstrap ci" (MR 2258)

.ci/mypy.sh

+#!/bin/sh -e
+# Description: static type checking for python scripts
+# https://postmarketos.org/pmb-ci
+
+if [ "$(id -u)" = 0 ]; then
+	set -x
+	apk -q add py3-argcomplete py3-mypy
+	exec su "${TESTUSER:-build}" -c "sh -e $0"
+fi
+
+set -x
+
+mypy pmbootstrap.py

.gitlab-ci.yml

@@ -59,7 +59,5 @@ mr-settings:
 
 mypy:
   stage: lint
-  before_script:
-    - apk add py3-argcomplete py3-mypy
   script:
-    - mypy pmbootstrap.py
+    - ".ci/mypy.sh"