gh-36647: fix random doctest error in `src/sage/rings/polynomial/skew_polynomial_finite_field.pyx`
Fixes #36646.
Add `# random` tag to a doctest to avoid random failure reported in
#36646.
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URL: https://github.com/sagemath/sage/pull/36647
Reported by: David Coudert
Reviewer(s): Frédéric Chapoton
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