cmd/dist: join with TMPDIR more carefully to avoid // in path.
cmd/dist: join with TMPDIR more carefully to avoid // in path.

This might fix the mkdtemp problem on the darwin builders if they
have TMPDIR set to a path ending in a slash; at worse this will
result in cleaner path names.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/13097043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang