[WIP] installer: use ExecAndCaptureOutput

Innosetup 6.4 introduced a New function ExecAndCaptureOutput that
executes commands and captures there standard output and standard error
streams. We've been using our own howmgrown solution called
ExecWithCapture for this purpose, but it's a bit finicky. Switch to the
official implementation.

Signed-off-by: Matthias Aßhauer <[email protected]>

Author: rimrul