CI: use EngFlow's opal cluster for remote execution

[jayconrod]

Platform configurations and most flags copied from
github.com/EngFlow/example.

Fixes #31

Signed-off-by: Jay Conrod jay@engflow.com

[jayconrod]

cc @mateusz-olczyk FYI, this is a work in progress. Not working yet.

• The macOS build job is being picked up and is executing actions remotely, but is failing due to a permissions error in prepare_test_repo.py. Might be a remote execution readiness problem. The error message is shown in the console log.
• The bcr_test_module_macos job is failing due to an authentication issue. Probably a misconfiguration here on my parent.
• Linux jobs are not being picked up. These use a different type of CI runner, and the server needs to be configured to poll jobs from this repo. Not sure how to do this, I need to work with the internal team. This is an unusual configuration; as far as I can tell, other public repos that do this are misconfigured, so I don't want to copy them.

[mateusz-olczyk]

• The macOS build job is being picked up and is executing actions remotely, but is failing due to a permissions error in prepare_test_repo.py. Might be a remote execution readiness problem. The error message is shown in the console log.

The problem might come from here:

if file == BUILD_FILE_NAME:
    # If this is a BUILD.bazel file (after renaming), append a filegroup with the collected rules
    dest_file = out_root / "BUILD.bazel"
    shutil.copy2(src_file, dest_file)
    if rule_names := parse_rule_names(src_file.read_text()):
        append_filegroup(dest_file, rule_names)
        filegroup_labels.add(filegroup_label)

I think shutil.copy2 preserves the attributes of src_file. But in this if branch, we need to open the file in "append" mode. If the runner checks out files from the repo in read-only mode, then the code's logic is incorrect. You might want to change shutil.copy2 with something, which ensures "write" access.