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ARROW-8430: [CI] Configure self-hosted runners for Github Actions

Findings:
- we require recent git version >= 2.18
- no specific way to detect whether the job runs in a self hosted runner
- to use runner specific environment variable we can setup .bashrc but that requires an interactive bash session
- the setup-python action downloads prebuilt python distributions and only the official gha virtual environments are going to be supported
- we can use the preinstalled python on the runner but to setup archery we need to add ~/.local/bin to the path using the bashrc workaround described above
- in order to have the github actions cache working we cannot use named docker volumes but once we switch to ordinary docker volumes we have the permission issue where the mounted directories are written by the root user (on linux at least) ~, so I had to pass the docker user to match the host user and group, but then we don't have enough permission to create the /build directory within the container so I had to change the widely used build directory to /tmp~
- ~need to initialize `.docker` directory for volume caches before docker does with root permissions~

Closes #6512 from kszucs/self-hosted

Authored-by: Krisztián Szűcs <szucs.krisztian@gmail.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>