Make local file path unambiguous

pips detection of paths seems to be iffy with builder paths like
/b/cbuild/internal_master/infra_virtualenv/pip_packages, so we
explicitly mark this as a local file path.

BUG=chromium:674681
TEST=Reproduced error and rerun locally with fix

Change-Id: Id7526882d6fc02ec6ab8b92b1da3881d54605ff8
Reviewed-on: https://chromium-review.googlesource.com/419796
Commit-Ready: Allen Li <ayatane@chromium.org>
Tested-by: Allen Li <ayatane@chromium.org>
Reviewed-by: David Riley <davidriley@chromium.org>
diff --git a/create_venv b/create_venv
index 9f5f866..fc148e1 100755
--- a/create_venv
+++ b/create_venv
@@ -9,7 +9,7 @@
 set -eu
 
 basedir=$(readlink -f "$(dirname "${BASH_SOURCE[0]}" )")
-pkgdir="$basedir/pip_packages"
+pkgdir="file://$basedir/pip_packages"
 
 main() {
   if [[ $# -ne 2 ]]; then