Use enablerepo instead of no_rhsm to check custom repos

[r0x0d]

We were using the no_rhsm option to skip the check for custom repositories, and since the system can be unregistered or pre-registered, there is no need anymore to use no_rhsm in this action, instead, we can use enablerepo directly to validate if the repositories are valid or not.

Jira Issues:

-

Checklist

• [ ] PR has been tested manually in a VM (either author or reviewer)
• [ ] Jira issue has been made public if possible
• [ ] [RHELC-] or [HMS-] is part of the PR title
• [ ] GitHub label has been added to help with Release notes
• [ ] PR title explains the change from the user's point of view
• [ ] Code and tests are documented properly
• [ ] The commits are squashed to as few commits as possible (without losing data)
• [ ] When merged: Jira issue has been updated to Release Pending if relevant

---

TODO

• [ ] Create a Jira issue for this

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.83%. Comparing base (5c42c1f) to head (bcdb1b0).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1288 +/- ## ======================================= Coverage 95.83% 95.83% ======================================= Files 55 55 Lines 4756 4757 +1 Branches 836 836 ======================================= + Hits 4558 4559 +1 Misses 112 112 Partials 86 86 ``` | [Flag](https://app.codecov.io/gh/oamg/convert2rhel/pull/1288/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=oamg) | Coverage Δ | | |---|---|---| | [centos-linux-7](https://app.codecov.io/gh/oamg/convert2rhel/pull/1288/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=oamg) | `91.02% <100.00%> (+<0.01%)` | :arrow_up: | | [centos-linux-8](https://app.codecov.io/gh/oamg/convert2rhel/pull/1288/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=oamg) | `91.95% <100.00%> (+<0.01%)` | :arrow_up: | | [centos-linux-9](https://app.codecov.io/gh/oamg/convert2rhel/pull/1288/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=oamg) | `92.00% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=oamg#carryforward-flags-in-the-pull-request-comment) to find out more.

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