Makefile: use variables to point to the webserver

This makes it easier to add more rules that also point to the same server
and directory on that server.

No functional changes.

Makefile

@@ -30,6 +30,10 @@ FFMPEG_VERSION=5.0.1
 TOOLS=./tools
 TOOLS_DOWNLOAD=./tools/download
 
+# SSH account & hostname for publishing.
+WEBSERVER_SSH=flamenco@flamenco.blender.org
+WEBSERVER_ROOT=/var/www/flamenco.blender.org
+
 all: application
 
 # Install generators and build the software.
@@ -208,7 +212,7 @@ clean-webapp-static:
 project-website:
 	rm -rf web/project-website/public/
 	cd web/project-website; hugo --baseURL https://flamenco.blender.org/
-	rsync web/project-website/public/ flamenco@flamenco.blender.org:/var/www/flamenco.blender.org/ \
+	rsync web/project-website/public/ ${WEBSERVER_SSH}:${WEBSERVER_ROOT}/ \
 		-va \
 		--exclude v2/ \
 		--exclude .well-known/ \