bug#30299: [core-updates] shepherd fails tests on all systems except x86_64

[Mark H Weaver]

On core-updates, Hydra has been unable to successfully build 'shepherd'
on any system except x86_64-linux.  I can also report that on my
mips64el-linux GuixSD system, which is running something close to
'core-updates', I had to disable tests on shepherd in order to build it.
I don't know about aarch64-linux.

These are the tests that commonly fail:

  FAIL: tests/respawn.sh
  FAIL: tests/respawn-throttling.sh
  FAIL: tests/basic.sh

      Mark