[DEV] Fix resolve_once script bug with custom functions

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.73684% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 97.04%. Comparing base (82c503c) to head (cda9fc7).

Files	Patch %	Lines
.../ytdl_sub/config/validators/variable_validation.py	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #952 +/- ## ========================================== - Coverage 97.05% 97.04% -0.01% ========================================== Files 115 115 Lines 7324 7339 +15 ========================================== + Hits 7108 7122 +14 - Misses 216 217 +1 ```

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