cmake: support SKIP_DASHED_BUILT_INS

cmake: support SKIP_DASHED_BUILT_INS

Just like the Makefile-based build learned to skip hard-linking the
dashed built-ins in 179227d6e21 (Optionally skip linking/copying the
built-ins, 2020-09-21), this patch teaches the CMake-based build the
same trick.

Note: In contrast to the Makefile-based process, the built-ins would
only be linked during installation, not already when Git is built.
Therefore, the CMake-based build that we use in our CI builds _already_
does not link those built-ins (because the files are not installed
anywhere, they are used to run the test suite in-place).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>