meson: fix test-gstreamer

On Nix, test-gstreamer fails with

	36/38 test-gstreamer                          FAIL     0.07 s (exit status 1)

	--- command ---
	BUILT_WITH_MESON='1' /build/libnice/build/tests/nice-test-gstreamer
	--- stdout ---
	Running suite(s): nice_gstreamer_test
	0%: Checks: 1, Failures: 1, Errors: 0
	../libs/gst/check/gstcheck.c:466:F:nice:buffer_list_test:0: Could not create a 'nicesink' element
	Check suite udpsink ran in 0.002s (tests failed: 1)
	-------

Let’s add the build directory to GST_PLUGIN_SYSTEM_PATH_1_0, so that
the test can find the built GStreamer element.
2 jobs for meson-fixes in 1 minute and 53 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #72257
build autotools

00:01:03

passed #72258
build meson

00:01:53