fix(ncu-ci): fetch commits instead of PR metadata

[aduh95]

I was testing #911, I noticed I would get No commits found errors, and indeed it looks like I forgot to change the API call now that we're no longer relying on the PR metadata.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.70%. Comparing base (5ec915b) to head (9eabd4c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/pr_checker.js	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #915   +/-   ##
=======================================
  Coverage   79.70%   79.70%           
=======================================
  Files          39       39           
  Lines        4642     4642           
=======================================
  Hits         3700     3700           
  Misses        942      942           

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[aduh95]

/cc @nodejs/node-core-utils