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>