tzdata: disable network access

Without the change tests fail on darwin as
https://hydra.nixos.org/build/272249259/nixlog/164/tail:

    curl: (77) error setting certificate file: /no-cert-file.crt

Possibly because `stdenv` has `curl` there. Upstream suggests disabling
networking access with `CURL=:`. Let's use that.

(cherry picked from commit eff4e92)

pkgs/data/misc/tzdata/default.nix

@@ -34,6 +34,7 @@ stdenv.mkDerivation (finalAttrs: {
     "LIBDIR=${placeholder "dev"}/lib"
     "MANDIR=${placeholder "man"}/share/man"
     "AWK=awk"
+    "CURL=:" # disable network access
     "CFLAGS=-DHAVE_LINK=0"
     "CFLAGS+=-DZIC_BLOAT_DEFAULT=\\\"fat\\\""
     "cc=${stdenv.cc.targetPrefix}cc"