system_config.rb: show system Ruby info on Linux

[maxim-belkin]

I propose to report the version of system Ruby in config as this info might be helpful for debugging purposes.

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[MikeMcQuaid]

This should be Linux only (i.e. in os/extend) as we're not gonna use this on macOS.

[MikeMcQuaid]

Thanks for this @maxim-belkin!