runtime: fix TestLldbPython in module mode (again)
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 <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

cloudflare/go,josharian/go,CAFxX/go,josharian/go,akutz/go,CAFxX/go,nawawi/go,akutz/go,golang/go,nawawi/go,josharian/go,cloudflare/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,akutz/go,josharian/go,golang/go,josharian/go,golang/go,akutz/go,golang/go,akutz/go,josharian/go,golang/go,cloudflare/go,nawawi/go,akutz/go,akutz/go,CAFxX/go,CAFxX/go,cloudflare/go,akutz/go,cloudflare/go,cloudflare/go,CAFxX/go,josharian/go,josharian/go,nawawi/go,nawawi/go,nawawi/go,nawawi/go,golang/go,golang/go