makeFontsCache: Fix cross-compiling, missing fc-cache

This commit fixes missing fc-cache binary from make-fonts-cache.nix build:
```
builder for '/nix/store/az48nr8gdqrw3fliddmi82ghj2ljxrj4-fc-cache.drv' failed with exit code 127; last 1 log lines:
  /nix/store/p3z1lgsi7xymvl7akg531ikwiisqs4x5-stdenv-linux/setup: line 1299: fc-cache: command not found
cannot build derivation '/nix/store/swaxvjsf8h0rsmm9kigp6j3f5q5h4nvg-fc-00-nixos-cache.conf.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/wiaiv0pq7w1xm2i2fqp2ngd1ljb4n6n9-fontconfig-conf.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/4zhiwpiyccs0rs26bs3q0w8fwaxrrgw0-fontconfig-etc.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/xhvljdp9b00fbkapx6cbfs4sjdh49qwv-etc.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/w63q0n0vh7vkdfrjmhb41qy1alx7qa8s-nixos-system-nixos-19.09.git.c814289.drv': 1 dependencies couldn't be built
```