ws: Allow using cockpit-desktop with a custom cockpit-ws
ws: Allow using cockpit-desktop with a custom cockpit-ws

Make cockpit-desktop look at `$COCKPIT_WS` to use cockpit-ws from a
different path. This makes it trivial to test everything from the build
tree:

    COCKPIT_WS=./cockpit-ws ./cockpit-desktop /

mvollmer/cockpit,deryni/cockpit,andreasn/cockpit,cockpituous/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpituous/cockpit,mvollmer/cockpit,martinpitt/cockpit,andreasn/cockpit,andreasn/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,deryni/cockpit,mvollmer/cockpit,deryni/cockpit,andreasn/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,garrett/cockpit,cockpituous/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,deryni/cockpit,deryni/cockpit,garrett/cockpit,garrett/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,garrett/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,andreasn/cockpit,deryni/cockpit,martinpitt/cockpit,martinpitt/cockpit,andreasn/cockpit,cockpituous/cockpit,andreasn/cockpit,deryni/cockpit,garrett/cockpit,cockpituous/cockpit,mvollmer/cockpit,cockpituous/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit