musl and UClibc, the other Linux libcs supported in Nixpkgs, don't have NSS, so tcb can't be built for them.
Without the change build fails to find `gcc`: $ nix build --no-link -f. pkgsCross.riscv64.shadow > make[1]: Entering directory '/build/source/libs' > gcc -O2 -I../include -Wall -Wextra -fPIC -c libtcb.c -o libtcb.o > /nix/store/b9jxsncywlygxjjyql0z5bq6z29m6j4r-bash-5.1-p16/bin/bash: line 1: gcc: command not found