SI-7687 Handle spaces in %COMSPEC% path in scala.bat.

Double quoted %COMSPEC% to allow for spaces in the path to the
default interpreter (cmd.exe or equivalent).

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I manually tested before/after on Windows 7.
I also signed the CLA some time ago (this is my first contribution).

Review by... @retronym? (The pull request policy doesn't list anyone for that kind of things, so I don't know... :) )