Stop using meson.build_root

meson.build_root is deprecated, and gives the following warnings when building (now with meson 0.61.0).

Configuring test.src.gresource.xml using configuration
../../../../../../../../../jhbuild/checkout/gnome-maps/tests/meson.build:46: WARNING: Project targets '>= 0.61.0' but uses feature deprecated since '0.56.0': meson.build_root. use meson.project_build_root() or meson.global_build_root() instead.
Program glib-compile-resources found: YES (/usr/bin/glib-compile-resources)
../../../../../../../../../jhbuild/checkout/gnome-maps/tests/meson.build:53: WARNING: Project targets '>= 0.61.0' but uses feature deprecated since '0.56.0': meson.build_root. use meson.project_build_root() or meson.global_build_root() instead.