Guard against mismatched `--requirements-pex`. #2567

Job	Run time	Billable time
Check GitHub Organization	1s	0s
tox -e py312-pip24_0	22m 7s	0s
tox -e py312-pip24_0-integration	1h 3m 3s	0s
./dtox.sh -e py27-pip20	12m 18s	0s
tox -e format-check,lint-check,typecheck,vendor-check,package,docs	6m 11s	0s
./dtox.sh -e py37-pip20	16m 50s	0s
./dtox.sh -e py311-pip20	15m 29s	0s
./dtox.sh -e py311-pip22_3_1	17m 35s	0s
./dtox.sh -e py311-pip23_1_2	17m 7s	0s
./dtox.sh -e py312-pip24_0	15m 57s	0s
./dtox.sh -e py313-pip24_0_patched	15m 35s	0s
./dtox.sh -e pypy310-pip20	26m 38s	0s
./dtox.sh -e pypy310-pip22_3_1	28m 1s	0s
./dtox.sh -e pypy310-pip23_1_2	26m 56s	0s
./dtox.sh -e py27-pip20-integration	21m 42s	0s
./dtox.sh -e py37-pip22_3_1-integration	40m 47s	0s
./dtox.sh -e py37-pip23_1_2-integration	42m 16s	0s
./dtox.sh -e py311-pip20-integration	23m 52s	0s
./dtox.sh -e py311-pip22_3_1-integration	30m 7s	0s
./dtox.sh -e py311-pip23_1_2-integration	28m 18s	0s
./dtox.sh -e py312-pip24_0-integration	36m 2s	0s
./dtox.sh -e py313-pip24_0_patched-integration	40m 3s	0s
./dtox.sh -e pypy310-pip20-integration	47m 6s	0s
./dtox.sh -e pypy310-pip22_3_1-integration	55m 9s	0s
./dtox.sh -e pypy310-pip23_1_2-integration	53m 36s	0s
Gather Final Status	1s	0s
	11h 42m 47s	0s

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