test/system: Fix reading the os-release(5) VERSION_ID on Ubuntu

The current approach of extracting the VERSION_ID field from
os-release(5) assumes that the value is not quoted.  There's no
guarantee that this will be the case.  It only happens to be so on
Fedora by chance, and is different on Ubuntu:
  $ cat /etc/os-release
  ...
  VERSION_ID="22.04"
  ...

This means that "22.04", including the double quotes, is read as the
value of VERSION_ID on Ubuntu, not 22.04.  This is wrong because this
value can't be used as is in image and container names.  There's no
image called quay.io/toolbx/ubuntu-toolbox:"22.04" and double quotes are
not allowed in container names.

Instead, use the same approach as profile.d/toolbox.sh and the old POSIX
shell implementation that doesn't rely on the quoting of the
os-release(5) values.

Fallout from b27795a

containers#1320

test/system/libs/helpers.bash

@@ -520,7 +520,8 @@ function get_system_version() {
     return
   fi
 
-  echo $(awk -F= '/VERSION_ID/ {print $2}' $os_release | head -n 1)
+  . "$os_release"
+  echo "$VERSION_ID"
 }