Fix TestBuild script

Until our other way of managing dependencies gets integrated we
still need to download the dependencies at least once. Added
the necessary command to TestBuild.sh.

Change-Id: I6d15a18aa0583f5248530d09472f699d10c45aec
Fix running exiftool on Linux

- add new smart method FileLocator.LocateExecutable that also checks
  in ProgramFiles (on Windows) or in the folders included in the
  PATH environment variable (on Linux).
- add utility methods Platform.IsLinux/IsMono etc.
- on Linux, search for both "file.exe" and "file".

NOTE: since exiftool is a unmanaged binary we shouldn't add it
to the repo on Linux. Instead we rely on the installed package
version.

Change-Id: I552cdfb61b4d1f2cf639f0ed23fe9e4006187921