generate-secrets failed due to the `PYTHONPATH` env var that was set
through the nixops19_09 buildInput.
Fixes:
- Don't pull in the build environments of binaries that should be
available in PATH. Only extend PATH instead.
- Run generate-secrets in an empty environment
This avoids an extra delay and the unexpected creation of secrets when
run in another dir.
Needed for the 'fetch-release' script introduced in a later commit.
Instead of forking this repo, it is now recommended that users simply import the
nix-bitcoin module. This commit adds an example directory that contains the
network/ examples and a shell.nix for deployment with nixops.
