workflows: Run distcheck
workflows: Run distcheck

This lets us verify that our ability to do releases hasn't regressed.

Run this as part of the "valgrind" build, since we want to enable
gtk-doc for distcheck, and it's this build that already enables gtk-doc.
We don't want to do this in the main Autotools build, since
that enables AddressSanitizer, which often works badly with the
"scanner" tools in gtk-doc and GObject-Introspection - although this
is currently mitigated by --enable-asan not actually working as
intended (see #4844).

Signed-off-by: Simon McVittie <0e1aea601907c1a85c2dc96e5ea37e23c58d2e9b@collabora.com>

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak