Flags are not passed correctly to custom tasks

[george-hopkins]

When passing custom flags to custom tasks, the flags are not quoted correctly. I suppose we need to use quoteShell() here as well: https://github.com/nim-lang/nimble/blob/bdc9678e2211cebf843a6f470397d50077f7705f/src/nimblepkg/nimscriptwrapper.nim#L66