Stop running snapcraft with sudo (#8063)

* Do not use sudo when building the snap.

* add user to lxd

* Run with lxd group.

snap/local/build_and_install.sh

@@ -6,9 +6,15 @@ if [[ -z "$TRAVIS" ]]; then
     exit 1
 fi
 
+# Add the current user to the lxd group so they can run `snapcraft --use-lxd`
+# without sudo since running the command without sudo is required by newer
+# versions of snapcraft.
+sudo usermod -aG lxd "$USER"
 sudo /snap/bin/lxd.migrate -yes
 sudo /snap/bin/lxd waitready
 sudo /snap/bin/lxd init --auto
 tools/strip_hashes.py letsencrypt-auto-source/pieces/dependency-requirements.txt > constraints.txt
-sudo snapcraft --use-lxd
+# Run snapcraft with the lxd group since it has not been added to the current
+# shell.
+sg lxd -c 'snapcraft --use-lxd'
 sudo snap install --dangerous --classic *.snap