fix: --graph=foo.dot should not require graphviz installed (#10942)
### Description
I'm attempting to use `turbo ... --graph` in GHA to do some static
analysis of a very large repository. It's easy enough to extract
relevant patterns from a `.dot` file, but I wouldn't have expected to
need graphviz installed if I'm not actually using it.
I guess it turns out we _are_ using it, to do `dot - -T dot > out.dot`,
but this seems very odd when the `std::io::stdout()` case just outputs
directly. Seems like a bug.
### Testing Instructions
Edit your `PATH` to remove where graphviz is installed, then
```bash
turbo run build --graph foo.dot
```
Prior to this PR, you should get the error ` WARNING `turbo` uses
Graphviz to generate an image of your graph, but Graphviz isn't
installed on this machine.` and an empty `foo.dot`.
After this PR, you should end up with a populated dotfile and no
warning.
---------
Co-authored-by: Anthony Shew <anthonyshew@gmail.com>