Do allowed requisites check in stdenv/linux

Use the new allowedRequisites feature in stdenvLinux.

This way we properly check that the end-result stdenv of the quite
complicated multi-stage stdenvLinux building procedure is sane, and only
depends on the stuff that we know about.

Alternative would be to just disallowRequisites bootstrapTools, which is
the most common offender, but we have had other offenders in the past.

For these checks to actually fire, you currently have to use nixUnstable,
as the necessary feature will be released in Nix 1.8.