sudo PATH="$PATH" NIX_PATH="$NIX_PATH" `which nixos-install` --root /mnt
building the configuration in /mnt/etc/nixos/configuration.nix...
[196 built, 841 copied (3363.2 MiB), 0.8 MiB DL]
installing the boot loader...
setting up /etc...
Initializing machine ID from random generator.
perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
	LANGUAGE = (unset),
	LC_ALL = (unset),
	LANG = "fr_FR.UTF-8"
    are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").
Copied "/nix/store/3hmpbbcv1db42m9g34c9g4q6qinw50x4-systemd-237/lib/systemd/boot/efi/systemd-bootx64.efi" to "/boot/EFI/systemd/systemd-bootx64.efi".
Copied "/nix/store/3hmpbbcv1db42m9g34c9g4q6qinw50x4-systemd-237/lib/systemd/boot/efi/systemd-bootx64.efi" to "/boot/EFI/BOOT/BOOTX64.EFI".
Failed to move "/boot/loader/loader.conf" into place: Operation not permitted
Traceback (most recent call last):
  File "/nix/store/4svrqgsk67dhwmlj0jqphf4yj1v0yysq-systemd-boot-builder.py", line 210, in <module>
    main()
  File "/nix/store/4svrqgsk67dhwmlj0jqphf4yj1v0yysq-systemd-boot-builder.py", line 185, in main
    subprocess.check_call(["/nix/store/3hmpbbcv1db42m9g34c9g4q6qinw50x4-systemd-237/bin/bootctl", "--path=/boot", "install"])
  File "/nix/store/i1hh6prcyfqd066jdh4zwqfs58mqhdfw-python3-3.6.6/lib/python3.6/subprocess.py", line 291, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['/nix/store/3hmpbbcv1db42m9g34c9g4q6qinw50x4-systemd-237/bin/bootctl', '--path=/boot', 'install']' returned non-zero exit status 1.