Allow testing specific PR in Actions workflow

[dennisameling]

Closes #255

After merging this PR, then manually kicking off the API library tests, it will be possible to specify a specific PR from mautic/mautic to test against 🎉

Example run which tests against mautic/mautic#10259: https://github.com/mautic/api-library/actions/runs/1030643520

[codecov-commenter]

Codecov Report

Merging #256 (5019c00) into main (5cda363) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main     #256   +/-   ##
=========================================
  Coverage     51.45%   51.45%           
  Complexity      406      406           
=========================================
  Files            30       30           
  Lines          1028     1028           
=========================================
  Hits            529      529           
  Misses          499      499           

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[dennisameling]

mautic/mautic#10023 fixes the API library tests (see https://github.com/mautic/api-library/runs/3068060332?check_suite_focus=true), so this PR can be merged either before or after that PR has been merged 👍🏼

[escopecz]

Amazing! 👏

[RCheesley]

Thanks for the PR @dennisameling - super helpful feature! LGTM!