ci: make p4 setup on macOS more robust

When setting up Perforce on macOS we put both `p4` and `p4d` into "$HOME/bin". On GitHub CI this directory is indeed contained in the PATH environment variable and thus there is no need for additional setup than to put the binaries there. But GitLab CI does not do this, and thus our Perforce-based tests would be skipped there even though we download the binaries. Refactor the setup code to become more robust by downloading binaries into a separate directory which we then manually append to our PATH. This matches what we do on Linux-based jobs. Note that it may seem like we already did append "$HOME/bin" to PATH because we're actually removing the lines that adapt PATH. But we only ever adapted the PATH variable in "ci/install-dependencies.sh", and didn't adapt it when running "ci/run-build-and-test.sh". Consequently, the required binaries wouldn't be found during the test run unless the CI platform already had the "$HOME/bin" in PATH right from the start. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>