Update mkosi version in nix flake

[genjuro214]

We depend on nix to setup mkosi build environment. Current mkosi version in nix is 17.1:

# nix develop ..#podvm-mkosi --command mkosi --version
warning: Git tree '/root/cloud-api-adaptor' is dirty
mkosi 17.1

The mkosi version in nix has been updated to 20.2:
https://github.com/NixOS/nixpkgs/blob/nixos-unstable/pkgs/tools/virtualization/mkosi/default.nix#L48

We can run nix flake update to update mkosi version to 20.2 in nix.

However, we may hit some errors with mkosi 20.2:

• Environment option cannot work, need try profile instead:

nix develop ..#podvm-mkosi --command mkosi --profile debug.conf

• Failed to copy resources/binaries-tree:

‣ Building system image
‣  Copying in package manager file trees…
cp: cannot create directory '/root/.cache/mkosi-workspace08nm3do0/pkgmngr/usr/local': Read-only file system
cp: preserving permissions for '/root/.cache/mkosi-workspace08nm3do0/pkgmngr/usr': Read-only file system
‣ "cp --recursive --no-dereference --preserve=mode,links --reflink=auto /root/cloud-api-adaptor/src/cloud-api-adaptor/podvm-mkosi/resources/binaries-tree /root/.cache/mkosi-workspace08nm3do0/pkgmngr --no-target-directory" returned non-zero exit code 1.

I believe it's a bug that has been fixed in mkosi v21
workaround is to use ExtraTrees instead of SkeletonTrees in podvm-mkosi/mkosi.presets/system/mkosi.conf.d/fedora.conf

If we need update mkosi version in nix, we have to resolve the issues above, and verify the Pod VM image.