gst-uninstalled: Allow specifying the checkout directory by env variable

gst-uninstalled: Allow specifying the checkout directory by env variable

For some rare cases, one might not be able to use the hardcoded $HOME/gst
location yet would still want to use the gst-uninstalled script as-is (which
has the benefit of being constantly updated).

For these cases, the checkout directory can be specified with the
GST_UNINSTALLED_ROOT environment variable.

Ex:
export GST_UNINSTALLED_ROOT=$HOME/somewhere/with/checkouts

And then just call gst-uninstalled directly:
$GST_UNINSTALLED_ROOT/gstreamer/gst-uninstalled