fix: use BaseOS as variant for RHEL9.0 in Beaker

[f-trivino]

Only BaseOS variant is available for RHEL9.0 in Beaker.

[Tiboris]

We will need to use [ and ] to group 8 and 9

In [1]: host8 = {"os": "rhel-8"}

In [2]: host9 = {"os": "rhel-9"}

In [3]: import re

In [4]: re.match(r"(rhel-8|9)", host8["os"])
Out[4]: <re.Match object; span=(0, 6), match='rhel-8'>

In [5]: re.match(r"(rhel-8|9)", host9["os"]) # > nothing

In [6]: re.match(r"(rhel-[8|9])", host9["os"])
Out[6]: <re.Match object; span=(0, 6), match='rhel-9'>

In [7]: re.match(r"(rhel-[8|9])", host8["os"])
Out[7]: <re.Match object; span=(0, 6), match='rhel-8'>