bug#39926: Regression introduced by Shepherd 0.7.0 ('make check-system TESTS=btrfs-root-os' fails)

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

Hello,

Recently, I noticed that 'make check-system TESTS=btrfs-root-os' stopped
working at all, even on master without my changes.

So, I've git bisected the faulty commit, and got:

--8<---------------cut here---------------start------------->8---
git bisect start
# good: [35cb2ac56cce1496c622958b53e82ba5d0c0ec48] gnu: Add queen.
git bisect good 35cb2ac56cce1496c622958b53e82ba5d0c0ec48
# bad: [a46174f457f7e558e67b7103074ee9ba92ada5dc] build: file-systems: Specify a "nfsvers" option by default for NFS.
git bisect bad a46174f457f7e558e67b7103074ee9ba92ada5dc
# bad: [532b318020748c035b43f83611899e4ba0875ae1] gnu: Add r-xmisc.
git bisect bad 532b318020748c035b43f83611899e4ba0875ae1
# bad: [7046106ef90e7855311dbb1f1ae9fed3df5cf875] gnu: upx: Clean up.
git bisect bad 7046106ef90e7855311dbb1f1ae9fed3df5cf875
# good: [a9546f8bb1fde766dc8ccd435b03aac1f720bd0f] gnu: go-github-com-certifi-gocertifi: Update to 2018.01.18-1.a5e0173
git bisect good a9546f8bb1fde766dc8ccd435b03aac1f720bd0f
# good: [53ddc6a71f6d7968734007f0b5325c66d9c6a33b] gnu: emacs-markdown-mode: Use HTTPS home page.
git bisect good 53ddc6a71f6d7968734007f0b5325c66d9c6a33b
# good: [1788bb9016a98d9c9412bc1611156d33f3f731d0] gnu: openscenegraph: Update to 3.6.5.
git bisect good 1788bb9016a98d9c9412bc1611156d33f3f731d0
# bad: [35f985cb52d7bc3d7e03d5db0a63da4dcf4d3322] gnu: guile-present: Use HTTPS home page.
git bisect bad 35f985cb52d7bc3d7e03d5db0a63da4dcf4d3322
# good: [d39885a8a9e0e03c2bf6277d475d384168bba642] gnu: python-reno: Update home page.
git bisect good d39885a8a9e0e03c2bf6277d475d384168bba642
# bad: [21f5de6d32a7d0e75ec2df69dc753a75a15657cd] gnu: openssh: Don't use NAME in source URI.
git bisect bad 21f5de6d32a7d0e75ec2df69dc753a75a15657cd
# bad: [139a9887843d0ea7dd724c4dd0d00470337370b3] gnu: shepherd: Add "guile3.0-shepherd" variant.
git bisect bad 139a9887843d0ea7dd724c4dd0d00470337370b3
# bad: [205c1e04e04b9a9338c7219ff82bd13f000fb8c8] gnu: shepherd: Update to 0.7.0.
git bisect bad 205c1e04e04b9a9338c7219ff82bd13f000fb8c8
# first bad commit: [205c1e04e04b9a9338c7219ff82bd13f000fb8c8] gnu: shepherd: Update to 0.7.0.
--8<---------------cut here---------------end--------------->8---

So, it seems that updating shepherd to 0.7.0 caused this regression
(commit 205c1e04e04b9a9338c7219ff82bd13f000fb8c8).

It probably causes other tests to fail in (gnu tests install), but I
haven't tried any.

Maxim