CI: replace Flake8 with Ruff (MR 30)

See similar patch in pmbootstrap for rationale:
https://gitlab.com/postmarketOS/pmbootstrap/-/commit/27618d5ffdb0b6cbf9471dc01a2ed14e1655ced0

.ci/flake8.sh → .ci/ruff.sh

@@ -4,11 +4,11 @@
 
 if [ "$(id -u)" = 0 ]; then
 	set -x
-	apk -q add py3-flake8
+	apk -q add ruff
 	exec su "${TESTUSER:-build}" -c "sh -e $0"
 fi
 
 set -x
 
 # shellcheck disable=SC2046
-flake8 $(find .ci -name '*.py')
+ruff check $(find .ci -name '*.py')

.gitlab-ci.yml

 ---
 image: alpine:edge
 
-flake8:
+ruff:
   before_script:
+  - echo 'https://dl-cdn.alpinelinux.org/alpine/edge/testing' >> /etc/apk/repositories
   - adduser -D build
   script:
-  - .ci/flake8.sh
+  - .ci/ruff.sh
 mypy:
   before_script:
   - adduser -D build