nixos/doc: suggest mounting the ESP on /boot with umask=077

This prevents world-readable access to /boot, which is a security issue
that systemd-boot warns about.

Fixes https://github.com/NixOS/nixpkgs/issues/279362.

nixos/doc/manual/installation/installing.chapter.md

···
 
     ```ShellSession
     # mkdir -p /mnt/boot
-    # mount /dev/disk/by-label/boot /mnt/boot
+    # mount -o umask=077 /dev/disk/by-label/boot /mnt/boot
     ```
 
 3.  If your machine has a limited amount of memory, you may want to
···
 # mkfs.fat -F 32 -n boot /dev/sda3        # (for UEFI systems only)
 # mount /dev/disk/by-label/nixos /mnt
 # mkdir -p /mnt/boot                      # (for UEFI systems only)
-# mount /dev/disk/by-label/boot /mnt/boot # (for UEFI systems only)
+# mount -o umask=077 /dev/disk/by-label/boot /mnt/boot # (for UEFI systems only)
 # nixos-generate-config --root /mnt
 # nano /mnt/etc/nixos/configuration.nix
 # nixos-install