Support multiple override appends (#3261) #3622

Job	Run time
tox env type on ubuntu-latest	48s
test 3.12 on ubuntu-latest	4m 59s
tox env type on windows-latest	1m 25s
test 3.12 on windows-latest	9m 32s
tox env dev on ubuntu-latest	49s
test 3.12 on macos-latest	7m 46s
test 3.11 on ubuntu-latest	3m 13s
tox env dev on windows-latest	1m 23s
test 3.11 on windows-latest	8m 10s
tox env docs on ubuntu-latest	3m 4s
test 3.11 on macos-latest	6m 40s
tox env docs on windows-latest	6m 14s
test 3.10 on ubuntu-latest	3m 27s
tox env pkg_meta on ubuntu-latest	31s
test 3.10 on windows-latest	10m 56s
test 3.10 on macos-latest	6m 50s
test 3.9 on ubuntu-latest	3m 34s
test 3.9 on windows-latest	13m 25s
test 3.9 on macos-latest	6m 40s
test 3.8 on ubuntu-latest	3m 37s
test 3.8 on windows-latest	11m 50s
test 3.8 on macos-latest	6m 56s
	2h 1m 49s

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