cmake: better support for out-of-tree builds follow-up

cmake: better support for out-of-tree builds follow-up

In 7e0730c8baa (t: better support for out-of-tree builds, 2024-12-06),
the `bin-wrappers/` strategy was changed so that it no longer hard-codes
the template directory to be `@BUILD_DIR@/templates/blt`, but instead
interpolates the `@TEMPLATE_DIR@` placeholder during the build.

However, this commit only adjusted the `Makefile`-based build.

Let's adjust the CMake-based build as well. This fixes t0000.15 which
would otherwise fail with:

  ++ echo ''\''t1234-verbose/err'\'' is not empty, it contains:'
  't1234-verbose/err' is not empty, it contains:
  ++ cat t1234-verbose/err
  warning: templates not found in @TEMPLATE_DIR@

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