playground: Don't rely on /etc/hostname
playground: Don't rely on /etc/hostname

It does not exist on e. g. Fedora CoreOS. Call `hostname` in the
playground page just like the corresponding test
(`testShellPreload`) already does, to ensure they will be identical.

Closes #13136
cockpit-project/cockpit,cockpituous/cockpit,mvollmer/cockpit,deryni/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,deryni/cockpit,deryni/cockpit,garrett/cockpit,cockpituous/cockpit,cockpituous/cockpit,cockpituous/cockpit,deryni/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,cockpit-project/cockpit,cockpituous/cockpit,cockpit-project/cockpit,mvollmer/cockpit,deryni/cockpit,martinpitt/cockpit,garrett/cockpit,moraleslazaro/cockpit,deryni/cockpit,garrett/cockpit,moraleslazaro/cockpit,garrett/cockpit,deryni/cockpit,moraleslazaro/cockpit,garrett/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit