Apply maybeUnixPath to ssh path in build config

[Vigilans]

According to build ssh config's document:

ssh property syntax can be either:

default: Let the builder connect to the SSH-agent.
ID=path: A key/value definition of an ID and the associated path. It can be either a [PEM](https://en.wikipedia.org/wiki/Privacy-Enhanced_Mail) file, or path to ssh-agent socket.

Using a custom id myproject with path to a local SSH key:

build:
  context: .
  ssh:
    - myproject=~/.ssh/myproject.pem

SSH config's value type is the path to the key, therefore should be resolved with paths.ResolveRelativePaths.