snap: don't run snapcraft under sudo

Snapcraft 4.0 complains about running LXD builds under sudo.  So instead
add the CI user to the "lxd" group and use sg to switch to that
membership.

Resolves: #12712

.travis.yml

@@ -193,6 +193,7 @@ jobs:
         - LANG: C.UTF-8
         - SNAPCRAFT_ENABLE_SILENT_REPORT: y
         - SNAPCRAFT_ENABLE_DEVELOPER_DEBUG: y
+        - SNAPCRAFT_BUILD_ENVIRONMENT: lxd
       addons:
         snaps:
           - name: snapcraft
@@ -221,6 +222,7 @@ jobs:
       - LANG: C.UTF-8
       - SNAPCRAFT_ENABLE_SILENT_REPORT: y
       - SNAPCRAFT_ENABLE_DEVELOPER_DEBUG: y
+      - SNAPCRAFT_BUILD_ENVIRONMENT: lxd
   fast_finish: true
 
 before_install: ci/before_install.sh

ci/snap/install.sh

@@ -4,6 +4,7 @@ set -e
 set -o pipefail
 
 sudo apt update
+sudo usermod -aG lxd $USER
 sudo /snap/bin/lxd.migrate -yes
 sudo /snap/bin/lxd waitready
 sudo /snap/bin/lxd init --auto

ci/snap/script.sh

@@ -4,5 +4,5 @@ set -e
 set -o pipefail
 
 mkdir -p "$TRAVIS_BUILD_DIR/snaps-cache"
-sudo snapcraft --use-lxd
+sg lxd -c snapcraft