x/build: make sure builders always set $HOME and $USER

[bradfitz]

The builders don't always set $HOME and $USER.

Fix that, and revert the os/user testing hacks from https://go-review.googlesource.com/c/go/+/107300

[kardianos]

For all builders? or all non-windows builder?

[bradfitz]

$HOME/$USER or equivalent for windows/plan9. But we happen to be fine already for Windows & Plan 9, so this is really only about the Unix builders.

[gopherbot]

Change https://golang.org/cl/134218 mentions this issue: os/user: retrieve username from /etc/passwd instead of $USER