Fix section substitution with setenv #3751

Job	Run time
tox env type on ubuntu-latest	48s
test 3.12 on ubuntu-latest	4m 50s
tox env type on windows-latest	1m 19s
test 3.12 on windows-latest	13m 20s
tox env dev on ubuntu-latest	45s
test 3.12 on macos-latest	4m 57s
tox env dev on windows-latest	1m 17s
test 3.11 on ubuntu-latest	3m 19s
tox env docs on ubuntu-latest	2m 48s
test 3.11 on windows-latest	12m 14s
tox env docs on windows-latest	5m 11s
test 3.11 on macos-latest	3m 58s
tox env pkg_meta on ubuntu-latest	26s
test 3.10 on ubuntu-latest	3m 49s
test 3.10 on windows-latest	12m 48s
test 3.10 on macos-latest	4m 1s
test 3.9 on ubuntu-latest	3m 28s
test 3.9 on windows-latest	8m 26s
test 3.9 on macos-latest	4m 11s
test 3.8 on ubuntu-latest	3m 31s
test 3.8 on windows-latest	10m 55s
test 3.8 on macos-latest	5m 27s
	1h 51m 48s

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