libct: speedup process.Env handling #7275

Job	Run time
cross-i386	1m 2s
test (ubuntu-20.04, 1.22.x, rootless, -race)	7m 8s
test (ubuntu-20.04, 1.22.x, rootless)	6m 59s
test (ubuntu-20.04, 1.22.x, -race)	7m 47s
test (ubuntu-20.04, 1.22.x)	7m 10s
test (ubuntu-20.04, 1.23.x, rootless, -race)	7m 31s
test (ubuntu-20.04, 1.23.x, rootless)	7m 14s
test (ubuntu-20.04, 1.23.x, -race)	7m 47s
test (ubuntu-20.04, 1.23.x)	7m 6s
test (ubuntu-20.04, 1.23.x, criu-dev)	7m 54s
test (ubuntu-24.04, 1.22.x, rootless, -race)	9m 37s
test (ubuntu-24.04, 1.22.x, rootless)	9m 26s
test (ubuntu-24.04, 1.22.x, -race)	7m 33s
test (ubuntu-24.04, 1.22.x)	6m 55s
test (ubuntu-24.04, 1.23.x, rootless, -race)	9m 46s
test (ubuntu-24.04, 1.23.x, rootless)	9m 35s
test (ubuntu-24.04, 1.23.x, -race)	8m 5s
test (ubuntu-24.04, 1.23.x)	7m 14s
test (ubuntu-24.04, 1.23.x, criu-dev)	8m 4s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	11m 32s
test (actuated-arm64-6cpu-8gb, 1.23.x)	7m 55s
all-done	1s
	2h 43m 21s

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