jenkins: Build snap

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3678

.gitignore

@@ -16,3 +16,4 @@ syncthing.sig
 RELEASE
 deb
 lib/auto/gui.files.go
+snapcraft.yaml

jenkins/build-linux.bash

@@ -49,3 +49,8 @@ go run build.go -goarch armel deb
 go run build.go -goarch armhf deb
 
 mv *.deb "$WORKSPACE"
+
+go run build.go -goarch amd64 snap
+
+mv *.snap "$WORKSPACE"
+