Omit REQUESTS_CA_BUNDLE from build env defaults

[ryanskeith]

Description

Fixes #6063: stop injecting REQUESTS_CA_BUNDLE="" into the build environment when the variable is unset.

Changes

• Pass it through via get_defaultmechanism so it is only present when set in the host environment.
• Updated Document the inherited behavior; empty CA paths break TLS clients such as recent botocore (boto/botocore#3751).

Notes for review

• Left HTTP_PROXY / HTTPS_PROXY on the older os.getenv(..., "") path for now; they have the same pattern and could follow in a follow-up.
• REQUESTS_CA_BUNDLE is in os_vars so it applies on all platforms (including Windows requests/botocore).

Testing

• I was undecided if this should be made test.

Here is a LLM generated test used for local testing:

python - <<'PY'
import os, tempfile
from pathlib import Path
from conda_build.config import Config
from conda_build.metadata import MetaData
from conda_build.environ import get_dict

workdir = tempfile.mkdtemp()
recipe = Path(workdir) / "recipe"
recipe.mkdir()
(recipe / "meta.yaml").write_text(
    "package:\n  name: probe\n  version: 0.0.1\n"
    "build:\n  number: 0\n  noarch: generic\n"
    "about:\n  home: https://example.com\n  license: BSD-3-Clause\n  summary: probe\n"
)
m = MetaData(str(recipe), config=Config(croot=workdir, verbose=False, anaconda_upload=False))

os.environ.pop("REQUESTS_CA_BUNDLE", None)
assert "REQUESTS_CA_BUNDLE" not in get_dict(m=m), "unset should omit"

os.environ["REQUESTS_CA_BUNDLE"] = "/tmp/dummy-ca.pem"
assert get_dict(m=m)["REQUESTS_CA_BUNDLE"] == "/tmp/dummy-ca.pem", "set should pass through"
print("ok")
PY

[codspeed-hq]

Merging this PR will not alter performance

✅ 5 untouched benchmarks

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_{Comparing ryanskeith:fix-6063-requests-ca-bundle-passthrough (5a082be) with main (3b6a390)}