While it is possible to pass the passphrase via the environment, e.g.
`EnvironmentFile=<path to file that contains "RAD_PASSPHRASE=…">`
this is less secure than passing it via a file, because the environment
is inherited down the process tree.
Thus, allow using a systemd credential. The ID of the credential must be
xyz.radicle.node.passphrase
and is not user-configurable.
Passing the passphrase via file is now possible with
`LoadCredential=xyz.radicle.node.passphrase:<path to file that contains passphrase>`
This requires just a bit of plumbing in `radicle-node`.
Because this mechanism is more secure than using the environment
variable `RAD_PASSPHRASE`, it takes priority. That is, if both the
systemd credential is available, *and* the environment variable
`RAD_PASSPHRASE` is set, the former is preferred.
Heads-up:
1. The contents of the file must be valid UTF-8 (see documentation of
`std::fs::read_to_string`). Assuming that the passphrase is at some
point chosen by the user and typed on a keyboard, this does not
seem like a severe restriction.
2. The contents of the file are not processed otherwise, i.e. line
breaks (notably at the end of the file) are not stripped.
The related `issue/8bd040e9de05e7fc27e373ebc1649ff4ad930e7a` asked for a
very similar feature: Passing the passphrase via a file named by the
value of the of the environment variable `RAD_PASSPHRASE_FILE`.
It was also briefly discussed at
<https://radicle.zulipchat.com/#narrow/channel/369277-heartwood/topic/.60RAD_PASSPHRASE_FILE.60/with/529104447>.
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