Makefile: include SSH port in .env file

Include the SSH port in the `.env` file, so that we can rsync to non-
default port numbers.

.env.example

@@ -1,4 +1,5 @@
 # Copy this file as .env
 # SSH account & hostname for publishing.
 WEBSERVER_SSH=user@domain
+WEBSERVER_SSH_PORT=22
 WEBSERVER_ROOT=/path-to/flamenco.blender.org

Makefile

@@ -165,7 +165,7 @@ deploy-website:
 	rm -rf web/project-website/public/
 	go run ${HUGO_PKG} -s web/project-website --baseURL https://flamenco.blender.org/
 	rsync web/project-website/public/ ${WEBSERVER_SSH}:${WEBSERVER_ROOT}/ \
-		-e "ssh" \
+		-e "ssh -p ${WEBSERVER_SSH_PORT}" \
 		-rl \
 		--exclude v2/ \
 		--exclude downloads/ \