Fix unbound variable when printing an archive_contents_test failure message

[jszumski]

In the "standalone binary" branch ARCHIVE_ROOT was not set, which causes an error when printing the failure message of an archive_contents_test:

test/starlark_tests/verifier_scripts/archive_contents_test.sh: line 103: ARCHIVE_ROOT: unbound variable
-- Test log: -----------------------------------------------------------
(Log file did not exist.)
------------------------------------------------------------------------
 FAILED: Archive did not contain "does not exist" contents were:
 .

The xcarchive tests fall into this branch, so you can reproduce by adding a bogus string to it's contains array:

    archive_contents_test(
        name = "{}_contains_xcarchive_files_simulator".format(name),
        build_type = "simulator",
        target_under_test = "//test/starlark_tests/targets_under_test/ios:app_minimal.xcarchive",
        contains = [
            "does not exist",
        ],
        tags = [name],
    )