test/system: Support host operating systems without VERSION_ID

The VERSION_ID field in os-release(5) is optional [1].  It's absent on
Arch Linux, which follows a rolling-release model and uses the BUILD_ID
field instead:
  BUILD_ID=rolling

A subsequent commit will run the CI on Arch Linux.  Hence, the code to
get the default release from the host operating system can no longer
assume the presence of the VERSION_ID field in os-release(5).

Note that the arch-toolbox image is tagged with 'latest', in accordance
with OCI conventions, not 'rolling' [2,3], which is the os-release(5)
BUILD_ID.

A similar change was made to toolbox(1) in commits 2ee82af and
d14fd7b.

[1] https://www.freedesktop.org/software/systemd/man/os-release.html

[2] Commit 2568528
    2568528cb7f52663
    #861

[3] Commit a4e5861
    a4e5861ae5c93625
    #1308

#1438

test/system/libs/helpers.bash

@@ -560,7 +560,10 @@ function get_system_version() (
   # shellcheck disable=SC1090
   . "$os_release"
 
-  echo "$VERSION_ID"
+  local system_version="$VERSION_ID"
+  [ "$ID" = "arch" ] && system_version="latest"
+
+  echo "$system_version"
 )