GIT_WORK_TREE is not exported for post-checkout-hook

I have a post-checkout-hook shell script which indirectly calls
"git rev-parse --show-toplevel", and I've found that when that hook
runs for a checkout within a worktree directory, GIT_DIR is set but
GIT_WORK_TREE is not set, which leads to the CWD being returned
instead of the genuine toplevel value.

The script is seeing a GIT_DIR value like this:

    GIT_DIR=/path/to/repo/.git/worktrees/name

The only other git variables in the environment are GIT_EXEC_PATH
and GIT_PREFIX.

The outcome I'm seeing is explained by
https://git-scm.com/book/en/v2/Git-Internals-Environment-Variables#_repository_locations
which says:

    "If --git-dir or GIT_DIR is specified but none of --work-tree,
    GIT_WORK_TREE or core.worktree is specified, the current working
    directory is regarded as the top level of your working tree."

The current githooks(5) man page talks about this somewhat:

    "Environment variables, such as GIT_DIR, GIT_WORK_TREE, etc., are
    exported so that Git commands run by the hook can correctly locate
    the repository.  If your hook needs to invoke Git commands in a
    foreign repository or in a different working tree of the same
    repository, then it should clear these environment variables so
    they do not interfere with Git operations at the foreign
    location."

Unsetting GIT_DIR certainly fixes the problem, but I don't think that
ought to be necessary in my case, as my hook script is calling git in
the *same* working copy where the checkout happens.  It is not dealing
with "a foreign" repository [or] a different working tree".

My impression is that GIT_WORK_TREE should also be set for this hook
when dealing with a worktree.  I have tested this in git version 2.34
and 2.55 (maint) and also 2.56.0.rc0.355.g945ea7012d (next).

I have only tested post-checkout-hook, so I do not know if this
applies to other hooks as well.


[System Info]
git version:
git version 2.34.1
cpu: x86_64
no commit associated with this build
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
uname: Linux 6.8.0-138-generic #138~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Fri Aug  7 13:43:15 UTC  x86_64
compiler info: gnuc: 11.4
libc info: glibc: 2.35
$SHELL (typically, interactive shell): /bin/bash