verifier: humility isn't a tempfile

[iximeow]

@jordanhendricks just tripped over this as we snagging a cert from a racklette RoT. it turns out you run this from somewhere that can humility the system of interest, not on the system of interest, but getting an error about a TempFile(.. no such file or directory ..) was confusing!

I haven't actually run this (yet) but the types line up that failing to run the command would accidentally get reported as a tempfile thing unintentionally. wouldn't mind an actual check before merging (if Jordan has a sec?)

[flihp]

This is great, thanks @iximeow. Looks like I missed this case when converting this crate from anyhow to the ctrl plane's / john's error handling guide lines. I'll press the merge button to keep things moving.