Investigate --nix-run inconsistency

This does not work as expected:

```
❯ dapp --use solc:0.4.24 --nix-run solc solc --version
solc, the solidity compiler commandline interface Version: 0.5.3+commit.10d17f24.Linux.g++
```

Should probably return 0.4.24 instead - is this the sign of a bug in `--use`?