GITHUB_PATH always takes precedence over GITHUB_ENV

The docs for environment files say that you can write a key-value
pair to ${GITHUB_ENV} to “[update] an environment variable for any
actions running next in a job”. But this has an undesirable interaction
with the ${GITHUB_PATH} environment file: old GITHUB_PATH entries
are still included even after GITHUB_ENV is used to reset the variable
contents.

For instance, in this example…

    - run: echo '/some/path/dir' >>"${GITHUB_PATH}"
    - run: echo 'PATH=/usr/bin:/bin' >>"${GITHUB_ENV}"
    - run: echo "$PATH"

…we would expect the last step to print /usr/bin:/bin, because that
is the environment variable that has been explicitly set. But instead it
prints /some/path/dir:/usr/bin:/bin, using the path entries that were
prepended before the variable was cleared.

This is problematic because it makes it impossible to reset PATH after
a previous step has echoed to GITHUB_PATH. Specifically, I want to use
the setup-python action to configure a Python toolchain, then create a
virtualenv, and then remove the stock Python toolchain from the path—but
since setup-python uses ${GITHUB_PATH}, there is no way to do this.

(As a user, it’s hard to tell what part of the platform is responsible
for this behavior and whether that code is open source. I’ve filed
against this repository because setup-python uses core.addPath and
thus runs into this issue. If this isn’t the right place, please advise
me where I should file an issue instead. Thanks!)

Start at the core.addPath entry point and the linked environment-files documentation, then compare how setup-python writes GITHUB_PATH with later GITHUB_ENV updates. Determine whether this repository owns the behavior or whether it belongs to the GitHub Actions platform; done means identifying the responsible component and the expected reset behavior.