fetchdarcs: Use NIX_SSL_CERT_FILE

`SSL_CERT_FILE` has been replaced with `NIX_SSL_CERT_FILE`.

Before this change using `fetchdarcs` resulted in an error message like:

```
Identifying repository http://hub.darcs.net/scravy/easyplot inventory

darcs failed:  Not a repository: http://hub.darcs.net/scravy/easyplot (Peer certificate cannot be authenticated with given CA certificates)

HINT: Do you have the right URI for the repository?

builder for ‘/nix/store/imyvcs6lvb5yva66krc5wk39931sam8v-fetchdarcs.drv’ failed with exit code 2
```