Regularize dogfood PEX cache use. #3058

Job	Run time
Check GitHub Organization	1s
tox -e format-check,lint-check,typecheck,vendor-check,package,docs	8m 33s
tox -e py312-pip24_2	13m 19s
./dtox.sh -e py27-pip20	8m 20s
tox -e py312-pip24_2-integration --shard 1/2	13m 40s
./dtox.sh -e py37-pip22_3_1	10m 37s
tox -e py312-pip24_2-integration --shard 2/2	13m 38s
./dtox.sh -e py311-pip23_3_2	11m 58s
./dtox.sh -e py312-pip24_2	10m 34s
./dtox.sh -e py313-pip24_2	9m 19s
./dtox.sh -e pypy310-pip24_2	14m 2s
./dtox.sh -e py27-pip20-integration	26m 8s
./dtox.sh -e py37-pip22_3_1-integration --shard 1/2	12m 38s
./dtox.sh -e py37-pip22_3_1-integration --shard 2/2	17m 27s
./dtox.sh -e py311-pip23_3_2-integration --shard 1/2	21m 26s
./dtox.sh -e py311-pip23_3_2-integration --shard 2/2	22m 12s
./dtox.sh -e py312-pip24_2-integration --shard 1/2	19m 58s
./dtox.sh -e py312-pip24_2-integration --shard 2/2	21m 12s
./dtox.sh -e py313-pip24_2-integration --shard 1/2	18m 20s
./dtox.sh -e py313-pip24_2-integration --shard 2/2	19m 6s
./dtox.sh -e pypy310-pip24_2-integration --shard 1/2	12m 18s
./dtox.sh -e pypy310-pip24_2-integration --shard 2/2	12m 19s
Gather Final Status	1s
	5h 17m 6s

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