Let drake_visualizer work outside sandbox on Mac

[jamiesnape]

See also #5621 (comment).

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This change is 

[patmarion]

minor comment---some people might use commands like drake-visualizer --script relative/path/to/script.py which doesn't work if the launcher changes the current working directory.

[jamiesnape]

Indeed. Not a workflow we support on Linux either at the moment.

[sammy-tri]

I think this workflow may actually be encouraged in drake/multibody/rigid_body_plant/visualization/README (which uses relative paths to scripts)

[jamiesnape]

Yes, that would need to be modified for the current setup. This PR is just syncing up with the Linux behavior for now to meet a request by @RussTedrake in #5621. I am sure we can iterate on these scripts to support the workflow eventually.

[jamiesnape]

+@jwnimmer-tri for feature platform review.

[jwnimmer-tri]

platform review.

We should find someone with native OSX to test this as feature review, perhaps +@soonho-tri?

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Reviewed 1 of 1 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[soonho-tri]

We should find someone with native OSX to test this as feature review, perhaps +@soonho-tri?

I'll do that today.

[soonho-tri]

It works for me.

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Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable