libct: speedup process.Env handling #7226

Job	Run time
cross-i386	53s
test (ubuntu-20.04, 1.22.x, rootless, -race)	6m 56s
test (ubuntu-20.04, 1.22.x, rootless)	6m 59s
test (ubuntu-20.04, 1.22.x, -race)	7m 53s
test (ubuntu-20.04, 1.22.x)	7m 11s
test (ubuntu-20.04, 1.23.x, rootless, -race)	7m 7s
test (ubuntu-20.04, 1.23.x, rootless)	6m 47s
test (ubuntu-20.04, 1.23.x, -race)	7m 49s
test (ubuntu-20.04, 1.23.x)	7m 1s
test (ubuntu-20.04, 1.23.x, criu-dev)	7m 31s
test (ubuntu-24.04, 1.22.x, rootless, -race)	9m 46s
test (ubuntu-24.04, 1.22.x, rootless)	9m 18s
test (ubuntu-24.04, 1.22.x, -race)	7m 39s
test (ubuntu-24.04, 1.22.x)	6m 44s
test (ubuntu-24.04, 1.23.x, rootless, -race)	9m 58s
test (ubuntu-24.04, 1.23.x, rootless)	9m 38s
test (ubuntu-24.04, 1.23.x, -race)	8m 14s
test (ubuntu-24.04, 1.23.x)	7m 23s
test (ubuntu-24.04, 1.23.x, criu-dev)	8m 20s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	11m 29s
test (actuated-arm64-6cpu-8gb, 1.23.x)	7m 50s
all-done	0s
	2h 42m 26s

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