Unset CDPATH when running admin/configure in README

[agis]

On systems that set the CDPATH environment variable[1], cd might print the directory to stdout:

If cd uses a non-empty directory name from CDPATH, or if -
is the first argument, and the directory change is
successful, cd writes the absolute pathname of the new
working directory to the standard output.

This causes admin/lib/common.inc.bash to erroneously set MB_DOCKER_ROOT to `$'/home/musicbrainz-docker\n/home/musicbrainz-docker', which in turn causes the following error:

$ admin/configure with alt-db-only-mirror
admin/lib/common.inc.bash: line 12: cd: $'/home/musicbrainz-docker\n/home/musicbrainz-docker': No such file or directory
configure: fail to change directory to '/home/musicbrainz-docker
/home/musicbrainz-docker'

By temporarily unsetting CDPATH (just in case the user has set it), the command succeeds in my system.

[1] https://man7.org/linux/man-pages/man1/bash.1.html

[Copilot]

Pull request overview

Updates the installation instructions to avoid CDPATH-related stdout from cd breaking admin/configure on systems where CDPATH is set, improving reliability of the documented setup flow.

Changes:

• Prefixes the admin/configure with alt-db-only-mirror README command with CDPATH= to neutralize CDPATH for that invocation.

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