Bugfix value spec variables

[LiamHobby]

Fixed a bug where the presence of variables with VLM in the value_spec table would prevent variables of the same name being populated in the value_spec table from different datasets. The anti-join was performed on variable only - changed to c(variable, dataset).

[codecov]

Codecov Report

Attention: Patch coverage is 55.55556% with 8 lines in your changes missing coverage. Please review.

Project coverage is 85.44%. Comparing base (d4b3530) to head (eff75ce).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
R/spec_builder.R	38.46%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #118      +/-   ##
==========================================
- Coverage   85.64%   85.44%   -0.20%     
==========================================
  Files           8        8              
  Lines        1170     1175       +5     
==========================================
+ Hits         1002     1004       +2     
- Misses        168      171       +3     

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