cmd/go/internal/modfetch: make directories read-only after renaming, not before
The call to os.Rename was failing on the darwin builders, despite having passed in the TryBots.
(I tested this change by running 'go test cmd/go' manually on a darwin gomote.)
This fixes the builder failures after CL 146382.
Updates #26794
Fixes #29030
Change-Id: I3644773421789f65e56f183d077b4e4fd17b8325
Reviewed-on: https://go-review.googlesource.com/c/151798
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>