gstreamer: unbreak finding plugins in $NIX_PROFILES (#20207)

* gstreamer-1.0: make gst-launch find plugins again

gst-launch and friends are in the "dev" output now.

* gstreamer-1.0: lower priority on plugins from $NIX_PROFILES

Suffix the plugin paths from $NIX_PROFILES instead of prefixing them to
$GST_PLUGIN_SYSTEM_PATH. If a program has specifically set up its plugin
path to some custom/specific version, we don't want plugins from
$NIX_PROFILES to mess things up by having higher priority.