Add coder executable to PATH

[f0ssel]

Closes #1767

[AbhineetJain]

I wonder if the PATH env variable also works for Windows. Found that it does. Is there a way we can test this functionality?

[kylecarbs]

It's good that we're appending it to the path because that means it's the lowest priority. That would allow users to have a higher-priority Coder binary in the path without a problem.

It'd be helpful to add a comment explaining that. It could cause unexpected behavior if a user overrides it, but it's better than having a hard-to-override default.

[jsjoeio]

It's good that we're appending it to the path because that means it's the lowest priority. That would allow users to have a higher-priority Coder binary in the path without a problem.

non-blocking: should we add a test for that? i.e. user already has previous coder binary or coder v1 binary in path