Use GB rather than GiB for size

It's not exactly a nitpick. debos uses github.com/docker/go-units to
parse the size, and docker/go-units is fundamentally broken, as it
parses both GiB anb GB as GB. It's a known issue that's been there
forever, and will probably never be fixed because backward compat
(that's my guess).

Cf. <docker/go-units#31>, or better
<docker/go-units#27 (comment)>.

So let's make sure that we talk about GB, because that's what happens in
practice.

build.sh

@@ -45,7 +45,7 @@ Options:
   -b BRANCH   Kali branch used to build the image, default: $BRANCH
   -d DESKTOP  Desktop environment installed in the image, default: $DESKTOP
   -m MIRROR   Mirror used to build the image, default: $MIRROR
-  -s SIZE     Size of the disk image created in GiB, default: $SIZE
+  -s SIZE     Size of the disk image created in GB, default: $SIZE
   -t TYPE     Type of image to build (see below for details), default: $TYPE
   -v VERSION  Release version of Kali, defaults: ${BRANCH#kali-}
 
@@ -89,8 +89,8 @@ echo $SUPPORTED_DESKTOPS | grep -qw $DESKTOP \
 echo $SUPPORTED_TYPES | grep -qw $TYPE \
     || fail "Unsupported type '$TYPE'"
 
-[[ $SIZE =~ ^[0-9]+$ ]] && SIZE=${SIZE}GiB \
-    || fail "Size must be given in GiB and must contain only digits"
+[[ $SIZE =~ ^[0-9]+$ ]] && SIZE=${SIZE}GB \
+    || fail "Size must be given in GB and must contain only digits"
 [ "$VERSION" ] || VERSION=${BRANCH#kali-}
 
 # Print a summary of the build options