Move the .uuid file to /boot/grub/YYYY-mm-dd-HH-MM-SS-00.uuid

To prevent the file from being accidentally missed when someone copies
the ISO's contents, let's not place it in a directory that starts with a
dot. Since all GRUB related files are in /boot/grub/, put it there too.

Instead of using a more unique UUID for the file name, use
`YYYY-mm-dd-HH-MM-SS-00.uuid` which matches the ISO's modification date
in UTC,i.e. its "UUID". If multiple ISOs would be generated in the exact
same second, the ISO 9660 modification date (i.e. its "UUID") would be
the same, so there would be not way to distinguish between the volumes
anyway. This also makes the file look less suspicious to the casual
glance.

grub/grub.cfg

@@ -29,7 +29,7 @@ if [ -z "${ARCHISO_UUID}" ]; then
     if [ -z "${ARCHISO_HINT}" ]; then
         regexp --set=1:ARCHISO_HINT '^\(([^)]+)\)' "${cmdpath}"
     fi
-    search --no-floppy --set=root --file '/.disk/%UUID_SEARCH_FILENAME%.uuid' --hint "${ARCHISO_HINT}"
+    search --no-floppy --set=root --file '%ARCHISO_SEARCH_FILENAME%' --hint "${ARCHISO_HINT}"
     probe --set ARCHISO_UUID --fs-uuid "${root}"
 fi