Fixed build tools paths

[ncave]

Fixed build tools paths to match fcs/build.fsx.

[brettfo]

This should stay because lex/yacc are specifically copied to artifacts/Bootstrap here for Windows and here for Linux.

[ncave]

@brettfo Should fcs/build.fsx be fixed to use artifacts/Bootstrap too, for consistency?

[baronfel]

This is a good catch because I was working on mono builds of the compiler and build libraries on my fork and ran into the same problem.

doop

[KevinRansom]

@ncave,
we will need a different fix for this. Either the fcs build you are using, needs to use lex from Bootstrap or it needs to take responsibility for building/deploying Lex/Yacc itself.

Thanks

kevin