Add support for age plugins

modules/age.nix

@@ -12,6 +12,8 @@ with lib; let
 
   ageBin = config.age.ageBin;
 
+  pluginPaths = concatMapStrings (x: ":" + "${x}/bin") config.age.pluginPackages;
+
   users = config.users.users;
 
   mountCommand =
@@ -63,6 +65,7 @@ with lib; let
 
   installSecret = secretType: ''
     ${setTruePath secretType}
+    export PATH="${pkgs.gnugrep}/bin:${pkgs.coreutils}/bin:@out@/sw/bin:/usr/bin:/bin:/usr/sbin:/sbin${pluginPaths}"
     echo "decrypting '${secretType.file}' to '$_truePath'..."
     TMP_FILE="$_truePath.tmp"
 
@@ -187,6 +190,13 @@ in {
         The age executable to use.
       '';
     };
+    pluginPackages = mkOption {
+      type = types.listOf types.package;
+      default = [];
+      description = ''
+        List of age plugins that should be available in $PATH during the build.
+      '';
+    };
     secrets = mkOption {
       type = types.attrsOf secretType;
       default = {};