build: clear GO111MODULE during make.bash etc

The standard build assumes the variable is unset.
Make it so, like we do for GOFLAGS, GOBIN, and so on.

Change-Id: I4ad5695f8021b08bd1a35dd99112970a813d247c
Reviewed-on: https://go-review.googlesource.com/c/149959
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/make.bash

@@ -64,6 +64,7 @@ set -e
 
 unset GOBIN # Issue 14340
 unset GOFLAGS
+unset GO111MODULE
 
 if [ ! -f run.bash ]; then
 	echo 'make.bash must be run from $GOROOT/src' 1>&2

src/make.bat

@@ -48,6 +48,7 @@ setlocal
 
 set GOBUILDFAIL=0
 set GOFLAGS=
+set GO111MODULE=
 
 if exist make.bat goto ok
 echo Must run make.bat from Go src directory.

src/make.rc

@@ -48,6 +48,7 @@ if(~ $1 -v) {
 }
 
 GOFLAGS=()
+GO111MODULE=()
 GOROOT = `{cd .. && pwd}
 if(! ~ $#GOROOT_BOOTSTRAP 1)
 	GOROOT_BOOTSTRAP = $home/go1.4

src/run.bash

@@ -21,6 +21,7 @@ export GOPATH
 unset CDPATH	# in case user has it set
 unset GOBIN     # Issue 14340
 unset GOFLAGS
+unset GO111MODULE
 
 export GOHOSTOS
 export CC

src/run.bat

@@ -18,6 +18,7 @@ set GOPATH=
 :: Issue 14340: ignore GOBIN during all.bat.
 set GOBIN=
 set GOFLAGS=
+set GO111MODULE=
 
 rem TODO avoid rebuild if possible
 

src/run.rc

@@ -11,5 +11,6 @@ GOPATH = () # we disallow local import for non-local packages, if $GOROOT happen
             # to be under $GOPATH, then some tests below will fail
 GOBIN = () # Issue 14340
 GOFLAGS = ()
+GO111MODULE = ()
 
 exec go tool dist test -rebuild $*