Prevent tinypilot.postinst from appending to Janus config (#1615)

There was a bug in the tinypilot.postinst where we were appending to an
existing janus.plugin.ustreamer.jcfg file instead of the intended
behavior of recreating the file fresh on each install.
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Author: mtlynch

debian-pkg/debian/tinypilot.postinst

@@ -110,12 +110,13 @@ fi
 
 # Populate TinyPilot's Janus settings file.
 readonly JANUS_PLUGIN_USTREAMER_CONFIG='/etc/janus/janus.plugin.ustreamer.jcfg'
-cat <<EOF >> "${JANUS_PLUGIN_USTREAMER_CONFIG}"
+cat <<EOF > "${JANUS_PLUGIN_USTREAMER_CONFIG}"
 memsink: {
     object = "tinypilot::ustreamer::h264"
 }
 EOF
 
+# Append audio settings if TC358743 support is enabled.
 if "${tc358743_overlay_enabled}" ; then
   cat <<EOF >> "${JANUS_PLUGIN_USTREAMER_CONFIG}"
 audio: {