[release-branch.go1.21] cmd/go/testdata/script: clear path in gotoolchain_issue66175

The gotoolchain_issue66175 script test would fail if a binary named
go1.21 or go1.22 was present in the system PATH. Clear the path at the
beginning of the test to make it independent of the system path.

For #67277
Fixes #67310

Change-Id: I6d9133aee5b20ab116e07b659e8ecf3eb8add4ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/584258
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
(cherry picked from commit a0a6026)
Reviewed-on: https://go-review.googlesource.com/c/go/+/584935
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>

Author: matloob

src/cmd/go/testdata/script/gotoolchain_issue66175.txt

@@ -1,5 +1,10 @@
 env TESTGO_VERSION=go1.14
 
+# Clear the path so this test doesn't fail if the system running it\
+# has a binary named go1.21 or go1.22 on its path.
+[GOOS:plan9] env path=
+[!GOOS:plan9] env PATH=
+
 # check for invalid toolchain in go.mod
 go mod init m
 go mod edit -go=1.14 -toolchain=go1.22