t: allow overriding build dir

t: allow overriding build dir

Our "test-lib.sh" assumes that our build directory is the parent
directory of "t/". While true when using our Makefile, it's not when
using build systems that support out-of-tree builds.

In commit ee9e66e4e7 (cmake: avoid editing t/test-lib.sh, 2022-10-18),
we have introduce support for overriding the GIT_BUILD_DIR by creating
the file "$GIT_BUILD_DIR/GIT-BUILD-DIR" with its contents pointing to
the location of the build directory. The intent was to stop modifying
"t/test-lib.sh" with the CMake build systems while allowing out-of-tree
builds. But "$GIT_BUILD_DIR" is somewhat misleadingly named, as it in
fact points to the _source_ directory. So while that commit solved part
of the problem for out-of-tree builds, CMake still has to write files
into the source tree.

Solve the second part of the problem, namely not having to write any
data into the source directory at all, by also supporting an environment
variable that allows us to point to a different build directory. This
allows us to perform properly self-contained out-of-tree builds.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>