runtime: fix TestLldbPython in module mode (again)

When run with GOPATH=/dev/null, go build fails:

$ GOPATH=/dev/null go test -run=TestLldbPython -v -count=1 runtime
=== RUN   TestLldbPython
--- FAIL: TestLldbPython (0.21s)
    runtime-lldb_test.go:169: building source exit status 1
        go: failed to create cache directory /dev/null/pkg/mod/cache: mkdir /dev/null: not a directory
FAIL
FAIL	runtime	0.220s

But run.bash sets GOPATH=/dev/null.

Fix this by setting GOPATH to the empty string before passing to 'go build'.

Fixes #31100

Change-Id: I573c4755d209e0c3eb26c20d4f7870c2961f2782
Reviewed-on: https://go-review.googlesource.com/c/go/+/169918
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/runtime/runtime-lldb_test.go

@@ -164,6 +164,7 @@ func TestLldbPython(t *testing.T) {
 	// disable it for this test.
 	cmd := exec.Command(testenv.GoToolPath(t), "build", "-gcflags=all=-N -l", "-ldflags=-compressdwarf=false", "-o", "a.exe")
 	cmd.Dir = dir
+	cmd.Env = append(os.Environ(), "GOPATH=") // issue 31100
 	out, err := cmd.CombinedOutput()
 	if err != nil {
 		t.Fatalf("building source %v\n%s", err, out)