Iterative envar expansion seems to be flaky

[alecthomas]

Changing the Python package to:

env = {
  PYTHONPYCACHEPREFIX: "${PYTHONUSERBASE}/cache",
  PATH: "${PYTHONUSERBASE}/bin:${PATH}",
}

Caused expansion to fail and Python binaries not to be discovered.

[quad]

This PR caused the failure: https://github.com/cashapp/hermit-packages/commit/84bf94147d0ca70533633f36fa4d34b97527f1ff