Merge branch 'ps/ci-test-with-jgit'

Merge branch 'ps/ci-test-with-jgit'

Tests to ensure interoperability between reftable written by jgit
and our code have been added and enabled in CI.

* ps/ci-test-with-jgit:
  t0612: add tests to exercise Git/JGit reftable compatibility
  t0610: fix non-portable variable assignment
  t06xx: always execute backend-specific tests
  ci: install JGit dependency
  ci: make Perforce binaries executable for all users
  ci: merge scripts which install dependencies
  ci: fix setup of custom path for GitLab CI
  ci: merge custom PATH directories
  ci: convert "install-dependencies.sh" to use "/bin/sh"
  ci: drop duplicate package installation for "linux-gcc-default"
  ci: skip sudo when we are already root
  ci: expose distro name in dockerized GitHub jobs
  ci: rename "runs_on_pool" to "distro"