upgrading install-nix-action to fix build

[tewfik-ghariani]

All nixops as well as nixops-plugins' builds are failing at the 'nix installation' part

Error: The `add-path` command is disabled. Please upgrade to using Environment Files or opt into unsecure command execution by setting the `ACTIONS_ALLOW_UNSECURE_COMMANDS` environment variable to `true`. For more information see: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/
Error: Unable to process command '::add-path::/nix/var/nix/profiles/default/bin' successfully.
Error: The `add-path` command is disabled. Please upgrade to using Environment Files or opt into unsecure command execution by setting the `ACTIONS_ALLOW_UNSECURE_COMMANDS` environment variable to `true`. For more information see: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/
Error: Unable to process command '::set-env name=NIX_PATH::/nix/var/nix/profiles/per-user/root/channels' successfully.
Error: The `set-env` command is disabled. Please upgrade to using Environment Files or opt into unsecure command execution by setting the `ACTIONS_ALLOW_UNSECURE_COMMANDS` environment variable to `true`. For more information see: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

Ref : https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

It was fixed per https://github.com/cachix/install-nix-action/pull/52 so we need to upgrade to v12

cc @adisbladis

[tewfik-ghariani]

Thanks @AmineChikhaoui !