Remove remaining uses of pkg/homedir

[SamWhited]

This PR was created to address build failures when updating buildkit to vendor a more recent version of docker which is not currently usable by this package.

- What I did

Remove uses of "github.com/docker/docker/pkg/homedir".Get.

- How I did it

Replaced it with os.UserHomeDir which was added in Go 1.12 and is already in use in other places in this package.

- How to verify it

Run the tests and ensure everything builds. Comments explain why errors were unhandled to prevent confusion later.

Previous behavior can be found here.

- Description for the changelog

n/a

[thaJeztah]

This was removed in #2101, but later reverted in #2111 (see the description there for reasons why)

/cc @tiborvass @tonistiigi

[SamWhited]

This is still removed from pkg/homedir in more recent versions of Docker, should removing that function be reverted there too? That would also solve my problem with updating docker in buildkit.

[tonistiigi]

I don't understand this. The function exists in moby master https://github.com/moby/moby/blob/master/pkg/homedir/homedir_unix.go#L21 . I think you just need to vendor the correct version.

[SamWhited]

That's odd, I could swear I was looking at master, and the commit appears to be right. I'll experiment and try to figure out what I did to cause the wrong version to be vendored. Thanks.

[SamWhited]

Turns out that Travis was picking up the "Linux" build constraint, but not "Unix" so the file where Get was defined was not being included. There was a comment that said that the tag osusergo was required when building statically, so I've added that. Unsure why it was working before, but not now.