apps: Hide page in beiboot mode

Installing more apps only makes sense in the classic "installed cockpit
packages" world. But we don't want it in a beiboot session, as
installing any cockpit-* extension would install cockpit-bridge as a
dependency, and that then completely changes the session experience --
up to the point that we don't even install cockpit-system for the
Overview.

Since we dropped the cockpit-pcp on-demand install from the metrics
page, this was the last page that installed cockpit-* packages. This
is a prerequisite for enabling beiboot mode for distro packages
(although not sufficient yet), and adding more packages to the
cockpit/ws container.

This cannot currently be integration-tested: We don't yet support
beibooting for the rpms/debs, and Fedora CoreOS + ws container doesn't
have PackageKit.

pkg/apps/manifest.json

@@ -1,5 +1,6 @@
 {
     "conditions": [
+        {"path-exists": "/usr/bin/cockpit-bridge"},
         {"path-exists": "/lib/systemd/system/packagekit.service"},
         {"path-not-exists": "/sysroot/ostree"}
     ],