scripts: sphinx-pre-install: improve distro detection check

scripts: sphinx-pre-install: improve distro detection check

The Arch-linux detection is hit by catting /etc/issue, whose
contents is (nowadays):

	Arch Linux \r (\l)

It sounds a little ackward to print such string, so,
instead, let's use the /etc/os-release file, with exists
on lots of distributions and should provide a more reliable
result.

We'll keep the old tests before it, in order to avoid possible
regressions with the other distros, although the new way should
probably work on all the currently supported distributions.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/472924557afdf2b5492ae2a48c5ecfae216d54e2.1586883286.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>