1.15 backports

commit 8c7727a9:

gst-uninstalled: Don't add validate plugins to GST_PLUGIN_PATH
Validate plugins are automatically scanned from GST_VALIDATE_PLUGIN_PATH
instead. Adding them to GST_PLUGIN_PATH causes race conditions as the
plugins may be loaded before validate itself.

I included this commit to make cherry-picking the more important commit easier:

commit f922969a:

gst-uninstalled: Fix compatibility with meson 0.50
Starting with Meson 0.50, meson instrospect --targets uses a list
for the filename and install_filenames of each target. Handle both
lists and strings.

And this simple quality-of-life commit:

commit a4ab574c:

gst-uninstalled.py: Look for both 'build' and '_build' for builddir

The other commits seemed too invasive to me to backport.