iso-image: Improve disk detection

This should help in rare hardware-specific situations where the root is
not automatically detected properly.

We search using a marker file. This should help some weird UEFI setups
where the root is set to `(hd0,msdos2)` by default.

Defaulting to `(hd0)` by looking for the ESP **will break themeing**. It
is unclear why, but files in `(hd0,msdos2)` are not all present as they
should be.

This also fixes an issue introduced with cb5c4fcd3c
where rEFInd stopped booting in many cases. This is because it ended up
using (hd0) rather than using the `search` which was happening
beforehand, which in turn uses (hd0,msdos2), which is the ESP.
Putting back the `search` here fixes that.

nixos/modules/installer/cd-dvd/iso-image.nix

@@ -182,6 +182,9 @@ let
     # Menu configuration
     #
 
+    # Search using a "marker file"
+    search --set=root --file /EFI/nixos-installer-image
+
     insmod gfxterm
     insmod png
     set gfxpayload=keep
@@ -252,6 +255,9 @@ let
   } ''
     mkdir -p $out/EFI/boot/
 
+    # Add a marker so GRUB can find the filesystem.
+    touch $out/EFI/nixos-installer-image
+
     # ALWAYS required modules.
     MODULES="fat iso9660 part_gpt part_msdos \
              normal boot linux configfile loopback chain halt \
@@ -383,8 +389,10 @@ let
     ${lib.optionalString (refindBinary != null) ''
     # GRUB apparently cannot do "chainloader" operations on "CD".
     if [ "\$root" != "cd0" ]; then
+      # Force root to be the FAT partition
+      # Otherwise it breaks rEFInd's boot
+      search --set=root --no-floppy --fs-uuid 1234-5678
       menuentry 'rEFInd' --class refind {
-        # \$root defaults to the drive the EFI is found on.
         chainloader (\$root)/EFI/boot/${refindBinary}
       }
     fi