Disable test_entropy for riscv 64 #1031

Job	Run time
Tests with OCaml 5 (5.2.1, macos-latest)	2m 17s
Tests with OCaml 5 (5.2.1, ubuntu-latest)	1m 26s
Tests (4.14.2, macos-latest)	4m 22s
Tests (4.14.2, ubuntu-latest)	2m 10s
	10m 15s

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