cacert: Add Haskell x509-system compatibility

This allows cacert to be used with Haskell-based fetchers like
you would with regular OpenSSL-based fetchers:

  buildInputs = [ cacert ];

pkgs/data/misc/cacert/setup-hook.sh

@@ -1,3 +1,7 @@
 export NIX_SSL_CERT_FILE=@out@/etc/ssl/certs/ca-bundle.crt
-# left for compatibility
+
+# compatibility
+#  - openssl
 export SSL_CERT_FILE=@out@/etc/ssl/certs/ca-bundle.crt
+#  - Haskell x509-system
+export SYSTEM_CERTIFICATE_PATH=@out@/etc/ssl/certs/ca-bundle.crt