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

2 files changed, 49 insertions, 4 deletions

diff --git a/pkgs/development/libraries/gnutls/default.nix b/pkgs/development/libraries/gnutls/default.nix
index 4c4094987dd..ace18afdeed 100644
--- a/pkgs/development/libraries/gnutls/default.nix
+++ b/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) ''
diff --git a/pkgs/development/libraries/gnutls/dummy.crt b/pkgs/development/libraries/gnutls/dummy.crt
new file mode 100644
index 00000000000..77300f6376b
--- /dev/null
+++ b/pkgs/development/libraries/gnutls/dummy.crt
@@ -0,0 +1,45 @@
+ACCVRAIZ1
+-----BEGIN CERTIFICATE-----
+MIIH0zCCBbugAwIBAgIIXsO3pkN/pOAwDQYJKoZIhvcNAQEFBQAwQjESMBAGA1UE
+AwwJQUNDVlJBSVoxMRAwDgYDVQQLDAdQS0lBQ0NWMQ0wCwYDVQQKDARBQ0NWMQsw
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+-----END CERTIFICATE-----