profile: bail out early if promptvars is disabled

We need promptvars, otherwise the prompt strings won't undergo parameter
expansion and we'd print them literally:

$ shopt -u promptvars
$ echo foo
$(__systemd_osc_context_ps0)foo

Resolves: #40620

profile.d/80-systemd-osc-context.sh

@@ -21,6 +21,10 @@
 # Treat missing $TERM same as "dumb".
 [ "${TERM:-dumb}" = "dumb" ] && return 0
 
+# We need promptvars, otherwise the prompt strings won't undergo parameter expansion
+# and we'd print them literally
+shopt -q promptvars || return 0
+
 __systemd_osc_context_escape() {
     # Escape according to the OSC 3008 spec. Since this requires shelling out
     # to 'sed' we'll only do it where it's strictly necessary, and skip it when