ci: twister: fix result merge when there's only 1 runner

The current test result merge command uses artifacts/*/*/twister.xml as
an argument but that directory hierarchy only works if there's >1
runners. If twister decies to only schedule one, then the only file is
going to be in

artifacts/twister-out/twister.xml

and the current command is going to fail with a file not found error.

Using a ** glob for finding the files.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>