Fix Oscar.build() #14861

Job	Run time
doctest (1.10, ubuntu-latest)	28m 33s
doctest (1.11-nightly, ubuntu-latest)	27m 40s
test (1.6, short, ubuntu-latest)	1h 9m 38s
doctest (nightly, ubuntu-latest)	28m 24s
doctest (1.10, ubuntu-latest, depwarn=error)	28m 12s
doctest (1.10, macOS, RPTU)	17m 38s
test (1.6, long, ubuntu-latest)	37m 21s
test (1.10, short, ubuntu-latest)	48m 27s
test (1.10, long, ubuntu-latest)	33m 32s
test (1.11-nightly, short, ubuntu-latest)	57m 34s
test (1.11-nightly, long, ubuntu-latest)	39m 4s
test (nightly, short, ubuntu-latest)	1h 5m 11s
test (nightly, long, ubuntu-latest)	38m 44s
test (1.10, short, ubuntu-latest, depwarn=error)	47m 46s
test (1.10, long, ubuntu-latest, depwarn=error)	40m 7s
test (1.10, short, macOS, RPTU)	18m 12s
test (1.10, long, macOS, RPTU)	17m 20s
test (1.10, book, ubuntu-latest)	31m 1s
	11h 14m 24s

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