cmd/go: in TestScript/gotoolchain_path, remove the user's PATH

This test checks a behavior of GOTOOLCHAIN when an appropriate
toolchain is found in PATH. That requires it to exclude any suitable
toolchain binaries from the user's $PATH, which may otherwise
interfere.

Fixes #62709.

Change-Id: Ie9161e52d33a65be0b5265cb49e9f2bc8473e057
Reviewed-on: https://go-review.googlesource.com/c/go/+/529217
Auto-Submit: Bryan Mills <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: Bryan C. Mills

src/cmd/go/testdata/script/gotoolchain_path.txt

@@ -8,11 +8,12 @@ env TESTGO_VERSION=go1.21pre3
 # Compile a fake toolchain to put in the path under various names.
 env GOTOOLCHAIN=
 mkdir $WORK/bin
-[!GOOS:plan9] env PATH=$WORK/bin${:}$PATH
-[GOOS:plan9] env path=$WORK/bin${:}$path
 go build -o $WORK/bin/ ./fakego.go  # adds .exe extension implicitly on Windows
 cp $WORK/bin/fakego$GOEXE $WORK/bin/go1.50.0$GOEXE
 
+[!GOOS:plan9] env PATH=$WORK/bin
+[GOOS:plan9] env path=$WORK/bin
+
 go version
 stdout go1.21pre3