nixos/modules/profiles/base.nix: omit zfs if unavailable

The `boot.zfs.enabled` option is marked `readOnly`, so this is the only way to
successfully build a NixOS installer image for platforms that zfs does not build
for.

Co-authored-by: Alyssa Ross <hi@alyssa.is>

nixos/modules/profiles/base.nix

@@ -51,7 +51,9 @@
   ];
 
   # Include support for various filesystems.
-  boot.supportedFilesystems = [ "btrfs" "reiserfs" "vfat" "f2fs" "xfs" "zfs" "ntfs" "cifs" ];
+  boot.supportedFilesystems =
+    [ "btrfs" "reiserfs" "vfat" "f2fs" "xfs" "ntfs" "cifs" ] ++
+    lib.optional (lib.meta.availableOn pkgs.stdenv.hostPlatform config.boot.zfs.package) "zfs";
 
   # Configure host id for ZFS to work
   networking.hostId = lib.mkDefault "8425e349";