refactor: update wait-for-pr step for improved consistency with other built-ins

[krancour]

This swaps the prNumberFromOpen field for prNumberFromStep, thereby not implying that the prNumber output this step consumes needs to have come from an git-open-pr step.

This improves consistency with the other built-in promotion steps, which use the same syntax to avoid implying that input needs to have come from a particular kind of step.

All this will be moot once we have some kind of EL built, but for now, this matters.

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[codecov]

Codecov Report

Attention: Patch coverage is 16.66667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 51.04%. Comparing base (794cfbc) to head (c47e21a).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/directives/git_pr_waiter.go	16.66%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2609   +/-   ##
=======================================
  Coverage   51.04%   51.04%           
=======================================
  Files         282      282           
  Lines       21056    21056           
=======================================
  Hits        10748    10748           
  Misses       9635     9635           
  Partials      673      673           

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