ci: Turn off PyPI release tests on pull requests

[matthewfeickert]

Description

At the moment, the PyPI release tests run on pull requests

pyhf/.github/workflows/release_tests.yml

Lines 1 to 4 in 9fbbbf9

	name: Current Release
	on:
	pull_request:

but this doesn't add much value and once we know that we've broken the public API there isn't much benefit in being reminded of this every PR. This PR just lets them continue to run nightly and on demand.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Turn off running PyPI release tests on pull requests given limited value add
* Use 'include' keyword over 'exclude' to reduce test matrix maintenance

[codecov]

Codecov Report

Merging #1664 (1901e8d) into master (9fbbbf9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1664   +/-   ##
=======================================
  Coverage   98.07%   98.07%           
=======================================
  Files          64       64           
  Lines        4216     4216           
  Branches      585      585           
=======================================
  Hits         4135     4135           
  Misses         48       48           
  Partials       33       33           

Flag	Coverage Δ
contrib	25.30% <ø> (ø)
doctest	61.05% <ø> (ø)
unittests	96.39% <ø> (ø)

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