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When there is not a nobody user (for example inside Docker), the
tests TestAmbientCaps and TestAmbientCapsUserns should be skipped
instead of failing.

This has been patched upstream in
https://github.com/golang/go/commit/1959703dddffef61760d5d4fdedc4df8d2d73438
and we use here that patch.

See merge request nexedi/slapos!1741