tests: fix meson test env setup to make sure we use the right gst-plugin-scanner
This is the same fix that was applied in gst-plugins-good in gst-plugins-good!603 and fixes the testsuite running in gst-build. Part-of: <!219>
This is the same fix that was applied in gst-plugins-good in gst-plugins-good!603 and fixes the testsuite running in gst-build. Part-of: <!219>