cmake: Fix pot-update target breakage when bash isn't the default shell

(Default shell as in the /bin/sh executable.)

On Debian and other systems using a different Bourne-compatible shell
that doesn't support some of Bash's syntax extensions (in particular,
'&>' as a redirection operator targeting stdout+stderr), the pot-update
target is completely broken, stuffing the POTFILES.in lists for either
the last (wesnoth-utbs) or first (wesnoth-ai) textdomains with paths for
every C/C++ source file from our tree.

Shells that do not interpret &> the way Bash does consider it to be the
stdout redirection operator following an asynchronous invocation. Of
course such a thing is bound to cause weird issues like this.