ci: fix p4d executable not being found on GitHub Actions

ci: fix p4d executable not being found on GitHub Actions

Our tests for git-p4(1) depend on the p4d(1) and p4(1) executables to
exist. As we require specific versions of those binaries which typically
aren't available on common distributions, we install them manually via
"ci/install-dependencies.sh".

This script will put the binaries into "$CUSTOM_PATH", which gets
defined by "ci/lib.sh" -- if not explicitly overridden, its value will
be set to "$HOME/path". This causes issues though when running our tests
as unprivileged user, as we do both in GitLab CI and GitHub Actions,
because "$HOME" will be different when installing dependencies and when
running the tests. Consequently, the downloaded binaries will not be
found unless "$CUSTOM_PATH" is overridden to a common location.

We already do this for GitLab CI, where it points to "/custom". Let's do
the same for GitHub Actions so that Perforce-based tests are executed
again.

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