Override openssl (1.0) w/ openssl 1.1.

openssl_1_1 in the pinned version of nixpkgs is 1.1.1c.

overlays.nix

 self: super: {
+  openssl = self.openssl_1_1;
+
   python27 = super.python27.override {
     packageOverrides = python-self: python-super: {
       # new tahoe-lafs dependency