cmake.bbclass: Add ${COREBASE}/scripts to CMAKE_FIND_ROOT_PATH

${COREBASE}/scripts contains a "git" wrapper disabling fakeroot/pseudo.
This patch allows CMake to find ${COREBASE}/scripts/git instead of
${HOSTTOOLS_DIR}/git. This is needed for git invocations during do_install,
since do_install is a fakeroot task, and otherwise all git commands fail
with "fatal: detected dubious ownership in repository ...".

I don't know how common it is for CMake projects to invoke git during the
install phase intentionally. It's probably more common to do this during
the configure phase. However, the install step may re-run the configure
step, if some dependencies changed.

In my case, this happened in incremental Yocto builds which reran
do_install and repopulated parts of the recipe-sysroot during that,
without first rerunning do_configure or do_compile. One of the dependencies
changed (but only changing a file in some unrelated sub-package of it which
was not even installed into the recipe-sysroot), causing the dependant's
recipe-sysroot to be repopulated during do_install and thus causing the
CMake project to be reconfigured during do_install.

Signed-off-by: Daniel Klauer <daniel.klauer@gin.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>