os/exec: make skipStdinCopyError a function instead of a variable

os/exec: make skipStdinCopyError a function instead of a variable

This makes clearer that skipStdinCopyError is always defined and never
overridden in tests.

Secondarily, it may also help reduce init-time work and allow the
linker and/or inliner to better optimize this package.

(Noticed while prototyping #50436.)

Change-Id: I4f3c1bc146384a98136a4039f82165ed106c14b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/401897
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>