gnutls: avoid the (check-time) cacert dependency

The point is to reduce rebuild amount when updating cacert/nss,
though at *this* point it remains quite high - before & after:

Estimating rebuild amount by counting changed Hydra jobs (parallel=unset).
  13109 x86_64-darwin
  21567 x86_64-linux
Estimating rebuild amount by counting changed Hydra jobs (parallel=unset).
  13109 x86_64-darwin
  17141 x86_64-linux

pkgs/development/libraries/gnutls/default.nix

@@ -1,6 +1,6 @@
 { config, lib, stdenv, fetchurl, zlib, lzo, libtasn1, nettle, pkg-config, lzip
 , perl, gmp, autoconf, automake, libidn, p11-kit, libiconv
-, unbound, dns-root-data, gettext, cacert, util-linux
+, unbound, dns-root-data, gettext, util-linux
 , guileBindings ? config.gnutls.guile or false, guile
 , tpmSupport ? false, trousers, which, nettools, libunistring
 , withSecurity ? false, Security  # darwin Security.framework
@@ -77,9 +77,9 @@ stdenv.mkDerivation rec {
   propagatedBuildInputs = [ nettle ];
 
   inherit doCheck;
-  # stdenv's `NIX_SSL_CERT_FILE=/no-cert-file.crt` broke tests with:
-  #   Error setting the x509 trust file: Error while reading file.
-  checkInputs = [ cacert ];
+  # stdenv's `NIX_SSL_CERT_FILE=/no-cert-file.crt` breaks tests.
+  # Also empty files won't work, and we want to avoid potentially impure /etc/
+  preCheck = "NIX_SSL_CERT_FILE=${./dummy.crt}";
 
   # Fixup broken libtool and pkg-config files
   preFixup = lib.optionalString (!isDarwin) ''

pkgs/development/libraries/gnutls/dummy.crt

@@ -0,0 +1,45 @@
+ACCVRAIZ1
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----