This changes the logic for the parsing of private keys, as already
done for the source-controller, so that it is able to recognize and
work with a wider range of key formats instead of returning a vague
error:
```console
$ flux bootstrap git [..]
✗ ssh: this private key is passphrase protected
```
A patch for this was already submitted and merged in `go-git/go-git`,
but is not made available in a release yet:
https://github.com/go-git/go-git/pull/298
Signed-off-by: Hidde Beydals <hello@hidde.co>
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