feat: immediately filter for relevant `scenario_source`

[jdhoffa]

Since the scenario_thresholds file now has multiple possible scenario_source's, it is important to quickly filter it for only the relevant source, otherwise strange bugs appear:

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 9.69%. Comparing base (4385412) to head (1b30c12).

Files	Patch %	Lines
R/prep_alignment_table.R	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #316 +/- ## ===================================== Coverage 9.69% 9.69% ===================================== Files 27 27 Lines 2187 2187 ===================================== Hits 212 212 Misses 1975 1975 ```

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[jdhoffa]

Note: testing locally, this fixes the bug that was preventing the alignment table from rendering (ignore the #debug lines)