Support `pex3 cache prune --older-than ...`. #3139

Job	Run time
Check GitHub Organization	1s
tox -e format-check,lint-check,typecheck,vendor-check,package,docs	8m 41s
tox -e py313-pip24_3	13m 44s
./dtox.sh -e py27-pip20	10m 32s
tox -e py313-pip24_3-integration --shard 1/2	24m 44s
./dtox.sh -e py38-pip22_3_1	17m 11s
tox -e py313-pip24_3-integration --shard 2/2	35m 13s
./dtox.sh -e py311-pip23_3_2	18m 52s
./dtox.sh -e py313-pip24_3	13m 57s
./dtox.sh -e py314-pip24_3	14m 23s
./dtox.sh -e pypy310-pip24_3	27m 49s
./dtox.sh -e py27-pip20-integration	27m 17s
./dtox.sh -e py38-pip22_3_1-integration --shard 1/2	30m 38s
./dtox.sh -e py38-pip22_3_1-integration --shard 2/2	27m 23s
./dtox.sh -e py311-pip23_3_2-integration --shard 1/2	27m 46s
./dtox.sh -e py311-pip23_3_2-integration --shard 2/2	29m 18s
./dtox.sh -e py313-pip24_3-integration --shard 1/2	25m 45s
./dtox.sh -e py313-pip24_3-integration --shard 2/2	27m 10s
./dtox.sh -e py314-pip24_3-integration --shard 1/2	24m 19s
./dtox.sh -e py314-pip24_3-integration --shard 2/2	26m 6s
./dtox.sh -e pypy310-pip24_3-integration --shard 1/2	43m 59s
./dtox.sh -e pypy310-pip24_3-integration --shard 2/2	47m 24s
Gather Final Status	1s
	8h 42m 13s

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