[release-branch.go1.15] build: set GOPATH consistently in run.bash, run.bat, run.rc

We used to clear GOPATH in all the build scripts.
Clearing GOPATH is misleading at best, since you just end up
with the default GOPATH (%USERPROFILE%\go on Windows).
Unless that's your GOROOT, in which case you end up with a
fatal error from the go command (#43938).

run.bash changed to setting GOPATH=/dev/null, which has no
clear analogue on Windows.

run.rc still clears GOPATH.

Change them all to set GOPATH to a non-existent directory
/nonexist-gopath or c:\nonexist-gopath.

For #45238.
Fixes #45239.

Change-Id: I51edd66d37ff6a891b0d0541d91ecba97fbbb03d
Reviewed-on: https://go-review.googlesource.com/c/go/+/288818


Trust: Russ Cox <rsc@golang.org>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
(cherry picked from commit bb6efb96)
Reviewed-on: https://go-review.googlesource.com/c/go/+/304773


Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Trust: Carlos Amedee <carlos@golang.org>