ci: Disable systemtap on musl CI build
It’s not needed, and is now failing with:
meson.build:2578:36: ERROR: Feature systemtap cannot be enabled: Cannot enable systemtap because dtrace feature is disabled
A full log can be found at /builds/GNOME/glib/_build/meson-logs/meson-log.txt
See https://gitlab.gnome.org/GNOME/glib/-/jobs/3901860
Signed-off-by: Philip Withnall pwithnall@gnome.org