Pack warns about valid `clear-env` key in `buildpack.toml`

[edmorley]

Summary

Similar to #2192, Pack CLI gives an incorrect warning about a valid buildpack.toml key, but this time it's the clear-env key, which is allowed in the spec:
https://github.com/buildpacks/spec/blob/main/buildpack.md#buildpacktoml-toml

---

Reproduction

Steps

1. Create a buildpack whose buildpack.toml contains:

   api = "0.11"
   
   [buildpack]
   id = "testing-buildpack"
   version = "0.0.0"
   clear-env = true

2. pack build --builder heroku/builder:24 --buildpack <path_to_that_buildpack>

Current behavior

Warning: Ignoring unexpected key(s) in descriptor for buildpack testing-buildpack: buildpack.clear-env
===> ANALYZING
...

Expected behavior

No warning about an unexpected clear-env key.

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Environment

pack info

Pack CLI v0.35.1
lifecycle 0.20.1

[edmorley]

@fumblehool Thank you for fixing this! :-)