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.

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>

src/run.bash

@@ -23,15 +23,7 @@ fi
 
 eval $(../bin/go env)
 export GOROOT   # The api test requires GOROOT to be set, so set it to match ../bin/go.
-
-# We disallow local import for non-local packages, if $GOROOT happens
-# to be under $GOPATH, then some tests below will fail.  $GOPATH needs
-# to be set to a non-empty string, else Go will set a default value
-# that may also conflict with $GOROOT.  The $GOPATH value doesn't need
-# to point to an actual directory, it just needs to pass the semantic
-# checks performed by Go.  Use $GOROOT to define $GOPATH so that we
-# don't blunder into a user-defined symbolic link.
-export GOPATH=/dev/null
+export GOPATH=/nonexist-gopath
 
 unset CDPATH	# in case user has it set
 export GOBIN=$GOROOT/bin  # Issue 14340

src/run.bat

@@ -18,9 +18,7 @@ setlocal
 
 set GOBUILDFAIL=0
 
-:: we disallow local import for non-local packages, if %GOROOT% happens
-:: to be under %GOPATH%, then some tests below will fail
-set GOPATH=
+set GOPATH=c:\nonexist-gopath
 :: Issue 14340: ignore GOBIN during all.bat.
 set GOBIN=
 set GOFLAGS=

src/run.rc

@@ -12,10 +12,9 @@ if(! test -f ../bin/go){
 
 eval `{../bin/go env}
 
-GOPATH = () # we disallow local import for non-local packages, if $GOROOT happens
-            # to be under $GOPATH, then some tests below will fail
-GOBIN = () # Issue 14340
-GOFLAGS = ()
-GO111MODULE = ()
+GOPATH=/nonexist-gopath
+GOBIN=() # Issue 14340
+GOFLAGS=()
+GO111MODULE=()
 
 exec ../bin/go tool dist test -rebuild $*