libct: speedup process.Env handling #7057

Job	Run time
cross-i386	3m 23s
test (ubuntu-20.04, 1.22.x, rootless, -race)	6m 56s
test (ubuntu-20.04, 1.22.x, rootless)	6m 44s
test (ubuntu-20.04, 1.22.x, -race)	7m 45s
test (ubuntu-20.04, 1.22.x)	6m 34s
test (ubuntu-20.04, 1.23.x, rootless, -race)	7m 39s
test (ubuntu-20.04, 1.23.x, rootless)	7m 6s
test (ubuntu-20.04, 1.23.x, -race)	7m 33s
test (ubuntu-20.04, 1.23.x)	7m 2s
test (ubuntu-20.04, 1.23.x, criu-dev)	7m 43s
test (ubuntu-24.04, 1.22.x, rootless, -race)	10m 28s
test (ubuntu-24.04, 1.22.x, rootless)	9m 13s
test (ubuntu-24.04, 1.22.x, -race)	6m 50s
test (ubuntu-24.04, 1.22.x)	8m 29s
test (ubuntu-24.04, 1.23.x, rootless, -race)	10m 54s
test (ubuntu-24.04, 1.23.x, rootless)	9m 41s
test (ubuntu-24.04, 1.23.x, -race)	7m 55s
test (ubuntu-24.04, 1.23.x)	7m 32s
test (ubuntu-24.04, 1.23.x, criu-dev)	8m 55s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	11m 33s
test (actuated-arm64-6cpu-8gb, 1.23.x)	7m 49s
all-done	1s
	2h 47m 45s

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