dashboard/coordinator: set USER for the -nocgo builder
dashboard/coordinator: set USER for the -nocgo builder

... since it already caught a problem and was fixed in
hg rev 0b99c2137ccc.

I pushed this live already just so don't get a whole column of
fail that might hide other issues. Real users have USER set
anyway. (Perhaps we should set it for all our builders?)

LGTM=adg
R=golang-codereviews
CC=adg, dave, golang-codereviews, minux
https://golang.org/cl/137180043

proppy/build,golang/build,evandbrown/build,golang/build,evandbrown/build,evandbrown/build,golang/build,golang/build,proppy/build,proppy/build,proppy/build