dev-libs/libjcat: better control over build-time Python used

dev-libs/libjcat: better control over build-time Python used

libjcat/meson.build calls a helper Python script which depends on
modules 'sys' (core, always present), 'xml' (core, controlled by USE=xml)
and 'pkg_resources' (from dev-python/setuptools) - using whatever Python
version /usr/bin/python3 points to, which may well provide neither of
the latter two modules. Make sure the helper script is invoked using the
Python interpreter used to run meson itself, as that one is guaranteed
to both provide XML support (the very same import line appears all over
the package mesonbuild) and have setuptools (it's in RDEPEND of
dev-util/meson).

Closes: https://bugs.gentoo.org/715670
Signed-off-by: Marek Szuba <marecki@gentoo.org>