Inspect os-release(5) to see whether we are in a SteamRT container/chroot

Implements most of https://phabricator.collabora.com/T16564 and about half of https://phabricator.collabora.com/T16069.

• tests: Replace SRT_TEST_ICD_SYSROOT with SRT_TEST_SYSROOT

  I'm about to add a second feature that needs to fake a sysroot for test coverage.

• Add minimal OS information from os-release(5)

  This doesn't include information about similar OSs (ID_LIKE), and also does not include a fallback to lsb_release or OS-specific files. However, it's enough to detect Steam Runtime containers.

• Add support for ID_LIKE os-release(5) field

  This one is a bit different because the raw field is a space-separated list. For convenience, srt_system_info_dup_os_id_like() can be asked to include the OS itself, for use in logic like this pseudocode:

    for id in info.dup_os_id_like(True):
        if id in special_cases:
            special_cases[id].run()
            break
    else:
        do_generic_behaviour()

• runtime: Use os-release(5) to detect Steam Runtime container/chroot

  Instead of inspecting the LD_LIBRARY_PATH runtime with _srt_runtime_check(), we open-code a small partial reimplementation of that function to inspect os-release(5) and provide information about a Steam Runtime container.

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Missing things:

• From T16564: doesn't look at /overrides to populate SRT_RUNTIME_ISSUES_NOT_USING_NEWER_HOST_LIBRARIES. I'm not sure whether that's really worth it, tbh.

• From T16069 (which is out of scope for T16564):

  • falling back to lsb_release

/cc @denittis @jpwhiting