Fix hyperv packer build, update iso hash,

[luckynum7]

Changes:

• README.md: change packer build examples based on Makefile
• nixos.auto.pkvars.hcl: update iso hash
• nixos.pkr.hcl: fix local var in hyperv-iso section

• scripts\builders\hyperv-iso.nix: comment out systemd-boot to fix an error mentioned in https://discourse.nixos.org/t/21-11-22-05-upgrade-questions-thread/19386/22, e.g.

  error: The option `system.build.installBootLoader' is defined multiple times.
     Only one bootloader can be enabled at a time. This requirement has not
     been checked until NixOS 22.05. Earlier versions defaulted to the last
     definition. Change your configuration to enable only one bootloader.
  
     Definition values:
     - In `/home/ae/.nix-defexpr/channels/nixpkgs/nixos/modules/system/boot/loader/grub/grub.nix': <derivation /nix/store/pc51mqw9l1ygs40gc4a6rbh54c80hsd9-install-grub.sh.drv>
     - In `/home/ae/.nix-defexpr/channels/nixpkgs/nixos/modules/system/boot/loader/systemd-boot/systemd-boot.nix': <derivation /nix/store/y47482lpjs4dv2q5dpxsv1phs3pm2crx-systemd-boot.drv>

Tips:

• iso hash can be checked with

  curl -L https://channels.nixos.org/nixos-22.05/latest-nixos-minimal-x86_64-linux.iso.sha256
  # or
  curl -L https://channels.nixos.org/nixos-22.05/latest-nixos-minimal-i686-linux.iso.sha256

[zimbatm]

looks good, thanks!

It would be nice to pull the ISO from immutable locations so the hashes don't get out of date.