cmd/go: avoid setting variables for '/' and ':' in TestScript subprocess environments

cmd/go: avoid setting variables for '/' and ':' in TestScript subprocess environments

Also simplify platform-dependent handling of the PATH variable,
to make it more like the existing platform-dependent handling for
HOME and TMPDIR.

Fixes #53671.

Change-Id: Ica2665d3f61988c66fb6982b9feb61ca48eced79
Reviewed-on: https://go-review.googlesource.com/c/go/+/416554
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>