cmake(mergetools): better support for out-of-tree builds

cmake(mergetools): better support for out-of-tree builds

In 7e0730c8baa (t: better support for out-of-tree builds, 2024-12-06)
the strategy was changed from letting `t7609-mergetool--lib.sh`
hard-code the directory where it expects to find the merge tools to
hard-coding that value in the placeholder `@GIT_TEST_MERGE_TOOLS_DIR@`
that is replaced during the build.

However, likely due to a copy/paste mistake (and reviewers missed this,
too), the CMake-based build was adjusted incorrectly, replacing that
placeholder not with the path to the merge tools, but with a Boolean
indicating whether to use a runtime-generated path prefix or not.

Let's fix that, addressing this CMake-build's symptom:

  Initialized empty Git repository in D:/a/git/git/t/trash directory.t7609-mergetool--lib/.git/
  ++ . true/vimdiff
  ./test-lib.sh: line 1021: true/vimdiff: No such file or directory

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