Add `run` subcommand for sigh (#3855)
Add `run` subcommand for sigh (#3855)

* Add `run` subcommand for sigh

This lets you add new scripts with less boilerplate (just add a new line
to the `scripts` variable). Scripts can be run via:

```
./tools/sigh run <script name> <rest of args>
```

(I'm going to be adding additional scripts to be invoked by bazel soon, so this makes that slightly easier.)

* Fix bug: !scriptPath

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