In my version of rustc.nix it uses the "stdenv.targetPlatform.config" variable, which doenst work for armv7l since it slightly differs from the actual value needed.
to be precise: stdenv results in "armv7l-unknown-linux-gnueabihf"
while the target spec wants "armv7-unknown-linux-gnueabihf"
The lowercase l is not there. Which is the reason why building rustc fails.