env: fix modification of lib/profile in Plan 9 image
env: fix modification of lib/profile in Plan 9 image

This change fixes a mistake in CL 50890 where
the lib/profile file was overwritten instead of
removing the calls to "exec rio".

Change-Id: I3037aa93fce74f22d128c9bbaee8ba240cb275a3
Reviewed-on: https://go-review.googlesource.com/58470
Reviewed-by: Sarah Adams <2b342d1c32491b4e7587e4a0e291a5f0f95d4167@google.com>

golang/build,golang/build,golang/build,golang/build