[release-branch.go1.15] cmd/go: fix get_update_unknown_protocol test

This test needs to run in GOPATH mode. It broke when a go.mod file was
added to github.com/golang/example. This change sets GO111MODULE=off,
which matches master since CL 255051.

Fixes #43861

Change-Id: I9ea109a99509fac3185756a0f0d852a84c677bf5
Reviewed-on: https://go-review.googlesource.com/c/go/+/285956
Trust: Jay Conrod <[email protected]>
Run-TryBot: Jay Conrod <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

Author: Jay Conrod

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

@@ -1,5 +1,6 @@
 [!net] skip
 [!exec:git] skip
+env GO111MODULE=off
 
 # Clone the repo via HTTPS manually.
 exec git clone -q https://github.com/golang/example github.com/golang/example
@@ -10,4 +11,4 @@ cd github.com/golang/example
 exec git remote set-url origin xyz://github.com/golang/example
 exec git config --local url.https://github.com/.insteadOf xyz://github.com/
 
-go get -d -u -f github.com/golang/example/hello
+go get -d -u -f github.com/golang/example/hello