home-manager: add backup overwrite option

When using the backupFileExtension option, if the backup file exists,
the activation process fails. This adds an option to instead overwrite
the old backup instead of failing.

nix-darwin/default.nix

@@ -28,6 +28,7 @@ in
               ${lib.optionalString (
                 cfg.backupFileExtension != null
               ) "export HOME_MANAGER_BACKUP_EXT=${lib.escapeShellArg cfg.backupFileExtension}"}
+              ${lib.optionalString cfg.overwriteBackup "export HOME_MANAGER_BACKUP_OVERWRITE=1"}
               ${lib.optionalString cfg.verbose "export VERBOSE=1"}
               exec ${usercfg.home.activationPackage}/activate --driver-version ${driverVersion} >&2
             ''}