Detect arch like distros in /etc/os-release check already

[JOJ0]

• Fixes #16
• Even though /etc/arch-release is checked for existence as a fallback and is present on an - up to date as of writing - Manjora system, for some reason the arch variable is not set to 1. This fixes it, since os-release contains ID_LIKE=arch on Manjaro.

[JOJ0]

It is mysterious to me why the test -f fallback does not work but this proves that the "earlier" check for os-release does work with the fix:

Before the fix the check results in "Unsupported platform" output:

$ mppinit

MusicPlayerPlus initialization has previously been performed
and there is an existing MusicPlayerPlus configuration.

Preserve existing, Overwrite existing, Skip ? (p/o/s) p

Initializing MusicPlayerPlus, please wait ...Unsupported platform

        No mpcplus release asset found for this platform ...
        No mpplus-bliss release asset found for this platform ...
...
...

With fix:

$ mppinit

MusicPlayerPlus initialization has previously been performed
and there is an existing MusicPlayerPlus configuration.

Preserve existing, Overwrite existing, Skip ? (p/o/s) p

Initializing MusicPlayerPlus, please wait ...
        Installing mpcplus ...
        Installing mpplus-bliss ...
...
...

[JOJ0]

For reference. These are the files on the system being tested on:

$ cat /etc/os-release 
NAME="Manjaro Linux"
PRETTY_NAME="Manjaro Linux"
ID=manjaro
ID_LIKE=arch
BUILD_ID=rolling
ANSI_COLOR="32;1;24;144;200"
HOME_URL="https://manjaro.org/"
DOCUMENTATION_URL="https://wiki.manjaro.org/"
SUPPORT_URL="https://forum.manjaro.org/"
BUG_REPORT_URL="https://docs.manjaro.org/reporting-bugs/"
PRIVACY_POLICY_URL="https://manjaro.org/privacy-policy/"
LOGO=manjarolinux
$
$ cat /etc/arch-release 
Manjaro Linux
$