In a layer.conf (or some other configuration file) of one of your layers:
-------------------------------------------------------------------------------------------
# BB_GIT_DEFAULT_DESTSUFFIX is introduced in OE-Core Whinlatter.
# Give it a suitable default value that works with Walnascar.
BB_GIT_DEFAULT_DESTSUFFIX ??= "git"

# Define a variable to use as the top directory for S. In Walnascar it is
# expected to be ${WORKDIR}, but in Whinlatter it is expected to be
# ${UNPACKDIR}.
S_TOPDIR ??= "${WORKDIR}"

# OE-Core Whinlatter moves the fetched Git repositories to "${UNPACKDIR}/${BP}".
# It also produces errors for recipes that set S to the old "${WORKDIR}/git".
# Use WORKDIR_GIT to be able to support both Walnascar and Whinlatter.
WORKDIR_GIT = "${S_TOPDIR}/${BB_GIT_DEFAULT_DESTSUFFIX}"
-------------------------------------------------------------------------------------------

In some configuration file that is only included when building with Whinlatter or later:
-------------------------------------------------------------------------------------------
S_TOPDIR = "${UNPACKDIR}"
-------------------------------------------------------------------------------------------