fix -script parsing

The ':' introduced a new scope, and we would always wait on the outside
`file`, stdin.

Change-Id: I5176a80abd40650a0167150ff5e996613ecc93e7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/206581
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Reed <reed@google.com>