ci: fix setup of custom path for GitLab CI

ci: fix setup of custom path for GitLab CI

Part of "install-dependencies.sh" is to install some binaries required
for tests into a custom directory that gets added to the PATH. This
directory is located at "$HOME/path" and thus depends on the current
user that the script executes as.

This creates problems for GitLab CI, which installs dependencies as the
root user, but runs tests as a separate, unprivileged user. As their
respective home directories are different, we will end up using two
different custom path directories. Consequently, the unprivileged user
will not be able to find the binaries that were set up as root user.

Fix this issue by allowing CI to override the custom path, which allows
GitLab to set up a constant value that isn't derived from "$HOME".

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