Use flatpak pip generator for gcovr
Instead of doing pip network install, use https://github.com/flatpak/flatpak-builder-tools/tree/master/pip.
Changes need to be done here https://gitlab.gnome.org/GNOME/nautilus/blob/master/build-aux/flatpak/org.gnome.Nautilus.json#L35 and remove https://gitlab.gnome.org/GNOME/nautilus/blob/master/build-aux/flatpak/org.gnome.Nautilus.json#L28 when we don't need it.
Edited by Carlos Soriano