ENVPATH is static when building sources

[rsgalloway]

it seems that ${ENVPATH} is only checked once in get_sources and therefore any environment that updates this value will not get picked up downstream when resolving environments.

for example, the dev.env stack file updates ENVPATH to insert ${DEPLOY_ROOT}/dev/env

$ envstack dev -r ENVPATH
ENVPATH=/mnt/pipe/dev/env:/mnt/pipe/prod/env

but any .env files there do not get reflected in the resolved environment, only in Python subprocesses that run envstack.init() first.