sshprovider: on Windows, ModeSocket might not be set on the ssh socket

[nicks]

I tested this with https://github.com/tilt-dev/tilt, which links in the SSHProvider code directly and calls buildkit with the API library

Before this change, ssh mounts failed with

could not parse ssh: [default]: failed to parse C:/Users/nick/AppData/Local/Temp/ssh-fEsqO14952/agent.14952: ssh: no key found

After this change, they worked correctly.

Fixes #914

[tonistiigi]

Weird that I tried a similar fix in #914 (comment) but got a permission error afterward that I couldn't figure out. If you have any idea what may have caused that (or steps how a binary should be configured) you may leave instructions here as well.

From Go issue it looks like there are some safer ways to detect socket on windows but I guess this may do as a fallback for now.

Missing DCO

[nicks]

Fixed the DCO!

I would not be surprised if there are two different bugs - one for the file mode, and an independent one with the permission error. I did not see the permission error 😬

Yes, if you want, I can patch in one of the suggested fixes from golang/go#33357, but wasn't sure which one was dependable across versions. I was hoping that since this codepath is heading towards a failure case anyway, it's ok for the check to be a guesstimate.