This may not be unset in some environments, so let's override it with the
default value.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2661>
perf_testenv.set('G_MESSAGES_DEBUG', 'GNOME Shell')
perf_testenv.set('GNOME_SHELL_DATADIR', data_builddir)
perf_testenv.set('GNOME_SHELL_BUILDDIR', src_builddir)
+perf_testenv.set('GNOME_SHELL_SESSION_MODE', 'user')
perf_testenv.set('SHELL_BACKGROUND_IMAGE', '@0@'.format(background_file))
perf_testenv.append('GI_TYPELIB_PATH', gvc_typelib_path, separator: ':')
perf_testenv.append('LD_LIBRARY_PATH', libgvc_path, separator: ':')