Add support for passing the key password non-interactively

[dmbaturin]

Right now the only way to sign something with a password-protected key is to enter the key password via stdin. That is inconvenient for use in scripts and CI pipelines.

This PR adds support for passing the password in an environment variable called MINISIGN_PASSWORD.

I'm not claiming this design is actually good — it's more of a starting point for a discussion. I'm happy to change any details if you have different ideas.

[jedisct1]

stdin is not necessarily the console.

minisign -Sm file.zip < password.txt

echo 'password` |  minisign -Sm file.zip

echo "${ENVIRONMENT_VARIABLE}" | minisign -Sm file.zip

work.

You can also not use a password at all with the -W option.