The PYTHON variable is used in Makefile (https://gitlab.gnome.org/GNOME/glib/-/blob/2.58.3/glib/Makefile.am#L463) to generate the shebang for script gtester-report. With component/defaults.cfg, we have PYTHON=python2.7 so the shebang become '#!python2.7' which is an error for building a package on Fedora/SUSE:
[ 3269s] *** ERROR: ./opt/slapos/parts/glib/bin/gtester-report has shebang which doesn't start with '/' (python2.7)
The simplest solution is to remove this tester script as it is not needed and have been removed in newer version of glib already. This patch can thus be reverted when upgrading glib version.