Revert "bringup: Use full path when creating custom_source file"

Produces CI error:

Checking
/data4-xfs/gh/actions-runner/_work/kdevops/kdevops/default/kdevops/guest
fs/custom_images/debian-13-generic-amd64-daily/SHA512SUMS
debian-13-generic-amd64-daily.raw: OK
.//scripts/bringup_guestfs.sh: line 46: $CUSTOM_INDEX: ambiguous
redirect
Custom virt-builder source:
/etc/virt-builder/repos.d/kdevops-custom-images-debian-13-generic-amd64-
daily.conf
Custom virt-builder index:
Custom image source:
/data4-xfs/gh/actions-runner/_work/kdevops/kdevops/default/kdevops/guest
fs/custom_images/debian-13-generic-amd64-daily/debian-13-generic-amd64-d
aily.raw
------------------------------------------------------------------
This is a rolling distribution release! To upgrade just do:

rm -rf
/data4-xfs/gh/actions-runner/_work/kdevops/kdevops/default/kdevops/guest
fs/custom_images/debian-13-generic-amd64-daily/debian-13-generic-amd64-d
aily.raw/*
rm -f
/etc/virt-builder/repos.d/kdevops-custom-images-debian-13-generic-amd64-
daily.conf
rm -f

Running guests always use their own copy. To rebuild your custom
base image from the custom image, also remove the base image:

rm -f
/data4-xfs/gh/actions-runner/_work/kdevops/kdevops/default/kdevops/guest
fs/base_images/debian-13-generic-amd64-daily.raw

This can always be done safely without affecting running guests.
------------------------------------------------------------------
Going to build index for debian-13-generic-amd64-daily ...
virt-builder-repository: error: the repository must contain an index
file when running in automated mode

If reporting bugs, run virt-builder-repository with debugging enabled
and include the complete output:

  virt-builder-repository -v -x [...]
Failed to build repository ...
make: *** [scripts/guestfs.Makefile:77: bringup_guestfs] Error 1

This reverts commit 5649a0d.

scripts/bringup_guestfs.sh

@@ -111,7 +111,7 @@ build_custom_image()
 	CUSTOM_IMAGE="${CUSTOM_IMAGE_DIR}/${OS_VERSION}.raw"
 	CUSTOM_IMAGE_OK="${CUSTOM_IMAGE_DIR}.ok"
 	CUSTOM_SOURCE="/etc/virt-builder/repos.d/kdevops-custom-images-${OS_VERSION}.conf"
-	CUSTOM_INDEX="$(realpath ${CUSTOM_IMAGE_DIR}/index)"
+	CUSTOM_INDEX="${CUSTOM_IMAGE_DIR}/index"
 
 	mkdir -p ${CUSTOM_IMAGE_DIR}