gstreamer: wrap gst-{launch,inspect,typefind} with GST_PLUGIN_SYSTEM_PATH

So that the tools become useable. The cool thing about wrapping them
like this (looping over $NIX_PROFILES) is that they will work on
non-NixOS systems too, given that $NIX_PROFILES is set correctly.

If you want the old (pure?) behaviour, just run gst-launch etc. with
empty $NIX_PROFILES.