go: add goPrivate option

This option configures the `GOPRIVATE` variable. See

   https://golang.org/cmd/go/#hdr-Module_configuration_for_non_public_modules

for more information.

PR #1126

modules/programs/go.nix

@@ -62,6 +62,18 @@ in {
         example = ".local/bin.go";
         description = "GOBIN relative to HOME";
       };
+
+      goPrivate = mkOption {
+        type = with types; listOf str;
+        default = [ ];
+        example = [ "*.corp.example.com" "rsc.io/private" ];
+        description = ''
+          The <envar>GOPRIVATE</envar> environment variable controls
+          which modules the go command considers to be private (not
+          available publicly) and should therefore not use the proxy
+          or checksum database.
+        '';
+      };
     };
   };
 
@@ -85,5 +97,9 @@ in {
       home.sessionVariables.GOBIN =
         builtins.toPath "${config.home.homeDirectory}/${cfg.goBin}";
     })
+
+    (mkIf (cfg.goPrivate != [ ]) {
+      home.sessionVariables.GOPRIVATE = concatStringsSep "," cfg.goPrivate;
+    })
   ]);
 }