github: Workaround glib/seccomp issue on Ubuntu impish
The ubuntu-latest VMs are currently based on 20.04 (focal). In focal,
libseccomp2 doesn't know about the close_range syscall[1], but
g_spawn_sync in impish tries to use close_range since it's defined in
glibc. That causes libseccomp2 to return EPERM as it does for any
unknown syscalls. g_spawn_sync carries on silently instead of falling
back to other means of setting CLOEXEC on open FDs. Eventually it causes
some tests to hang since once side of a pipe is never closed. Remove
this when libseccomp2 in focal is updated or glib in impish handles the
EPERM better.
1. https://bugs.launchpad.net/ubuntu/+source/libseccomp/+bug/
1944436
Fixes: #2495