Harden primer package caching against possible race condition #9474

Job	Run time
run / pypy-3.9 / Linux	3m 7s
run / 3.9 / Linux	3m 15s
run / pypy-3.10 / Linux	3m 15s
run / 3.10 / Linux	3m 14s
run / 3.11 / Linux	3m 16s
run / 3.12 / Linux	3m 14s
run / 3.13-dev / Linux	3m 13s
run benchmark / ${{ matrix.python-version }} / Linux	1s
process / coverage	1s
run / ${{ matrix.python-version }} / macOS	1s
run / ${{ matrix.python-version }} / Windows	1s
	22m 38s

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