Expose `OPAMCONFIRMLEVEL` as `unsafe-yes` #3257

Job	Run time
Hygiene	55s
Test (macos-latest, 5.1, false)	6m 11s
Test (ubuntu-latest, 5.1, false)	3m 19s
Test (windows-latest, 4.14, false)	3m 35s
Test (ubuntu-latest, ocaml-variants.5.1.0+options,ocaml-option-flambda, true)	3m 27s
	17m 27s

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