Use local buildcurl endpoint for Python runtimes

[crohr]

Summary

• replace the hosted Barebuild SSH compiler call with a buildcurl-compatible HTTP request through BUILDCURL_URL
• send the Python version, target, and install prefix without retrying failed compiler builds
• add executable request, extraction, and failure tests

Validation

• shellcheck -e SC2034 bin/steps/python bin/download-python-runtime
• rspec spec/local_buildcurl_spec.rb (3 examples, 0 failures)
• adversarial correctness review: clean

[crohr]

CI note: lint and check-changelog pass on the reviewed head. The three Hatchet integration jobs fail before exercising the change because HEROKU_API_KEY and HEROKU_API_USER are empty in this repository; every failure is a Heroku API 401 Unauthorized. Local executable coverage passes with 3 examples and 0 failures.

[crohr]

@codex review