fix: support py_cc_toolchain

[jbedard]

Changes are visible to end-users: yes/no

• Searched for relevant documentation and updated as needed: yes/no
• Breaking change (forces users to change their own code or config): yes/no
• Suggested release notes appear below: yes/no

Test plan

• Covered by existing test cases
• New test cases added
• Manual testing; please provide instructions so we can reproduce:

[aspect-workflows]

✨ Aspect Workflows Tasks

📅 Fri Jun 19 20:28:35 UTC 2026

❌ 2 failed tasks

• ❌ test (test-root-bazel-8) · ⏱ 2m 2s · 🐙 GitHub Actions · ☑️ Check
  💬 failed in test · Bazel test failed (1 test failed)
• ❌ test (test-root-bazel-9) · ⏱ 2m 51s · 🐙 GitHub Actions · ☑️ Check
  💬 failed in test · Bazel test failed (1 test failed)

✅ 5 successful tasks

• ✅ buildifier · ⏱ 22.2s · 🐙 GitHub Actions · ☑️ Check
  💬 Format complete (clean)
• ✅ gazelle · ⏱ 27.4s · 🐙 GitHub Actions · ☑️ Check
  💬 Gazelle complete (clean)
• ✅ test (test-e2e-bazel-8) · ⏱ 2m 20s · 🐙 GitHub Actions · ☑️ Check
  💬 Bazel test complete (150/150 passed)
• ✅ test (test-e2e-bazel-9) · ⏱ 3m 7s · 🐙 GitHub Actions · ☑️ Check
  💬 Bazel test complete (145/145 passed · 128 cached)
• ✅ test (test-examples-uv_pip_compile-bazel-8) · ⏱ 32.2s · 🐙 GitHub Actions · ☑️ Check
  💬 Bazel test complete (1/1 passed · 1 cached)

🔁 Reproduce

❌ test (test-root-bazel-8 · test-root-bazel-9)

aspect test -- //uv/private/uv_hub:snapshots_9_test

# vanilla bazel
bazel test -- //uv/private/uv_hub:snapshots_9_test

_{Install aspect: docs.aspect.build/cli/install}

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[github-actions]

py_binary startup benchmark

Version	Mean (ms)	Median (ms)	± stddev	vs BCR	vs main	Build (s)
BCR 1.11.5 (baseline)	294.828	293.121	±6.915	—	—	40.22
HEAD main	57.239	57.105	±0.858	-80.6%	—	12.53
This PR	57.223	56.867	±1.466	-80.6%	-0.0%	11.25

Measured with hyperfine --warmup 5 --runs 50 on Linux
Gate: PR vs HEAD main (threshold: 10%). BCR is shown only as a historical baseline.
Build time: cold bazel build //:bench with isolated output base, no disk cache.

sys.path quality

Version	sys.path entries	distinct site-packages roots	duplicate realpaths
BCR 1.11.5 (baseline)	6	1	0
HEAD main	7	2	0
This PR	7	2	0

sys.path quality measured by bench_syspath inside the assembled venv. Duplicate realpaths indicate symlink redundancy; many distinct site-packages roots suggest an inefficient venv layout.