Add CI step for building a Snap package

.gitlab-ci.yml

+---
+stages:
+  - build
+
+snap:
+  stage: build
+  image: ubuntu:22.04
+  script:
+    - apt update
+    - apt -y install snapcraft
+    - snap install lxd
+    - lxd init --auto
+    - snapcraft --use-lxd
+  artifacts:
+    paths:
+      - autopsy_*.snap