The "No change in requirements detected, installing from cache" message is sometimes shown incorrectly

[edmorley]

Currently if the cache was discarded due to a reason other than the requirements.txt file changing and the app uses pip + has a requirements.txt file, then the No change in requirements detected, installing from cache message is shown here incorrectly:

heroku-buildpack-python/bin/steps/python

Line 106 in dba9b86

output::step "No change in requirements detected, installing from cache"

This can be seen in the TODOs in these tests:

heroku-buildpack-python/spec/hatchet/python_version_spec.rb

Lines 418 to 423 in dba9b86

	# TODO: The output shouldn't say "installing from cache", since it's not.
	expect(clean_output(app.output)).to include(<<~OUTPUT)
	remote: -----> Python app detected
	remote: -----> Using Python 3.13 specified in .python-version
	remote: -----> Python version has changed from #{LATEST_PYTHON_3_9} to #{LATEST_PYTHON_3_13}, clearing cache
	remote: -----> No change in requirements detected, installing from cache

heroku-buildpack-python/spec/hatchet/stack_spec.rb

Lines 22 to 34 in dba9b86

	# TODO: The requirements output shouldn't say "installing from cache", since it's not.
	expect(clean_output(app.output)).to include(<<~OUTPUT)
	remote: -----> Python app detected
	remote: -----> No Python version was specified. Using the same version as the last build: Python 3.12.3
	remote: To use a different version, see: https://devcenter.heroku.com/articles/python-runtimes
	remote:
	remote: ! Warning: A Python security update is available!
	remote: !
	remote: ! Upgrade as soon as possible to: Python #{LATEST_PYTHON_3_12}
	remote: ! See: https://devcenter.heroku.com/articles/python-runtimes
	remote:
	remote: -----> Stack has changed from heroku-22 to heroku-24, clearing cache
	remote: -----> No change in requirements detected, installing from cache