libct: speedup process.Env handling #7220

Job	Run time
cross-i386	45s
test (ubuntu-20.04, 1.22.x, rootless, -race)	7m 6s
test (ubuntu-20.04, 1.22.x, rootless)	6m 52s
test (ubuntu-20.04, 1.22.x, -race)	1m 20s
test (ubuntu-20.04, 1.22.x)	1m 2s
test (ubuntu-20.04, 1.23.x, rootless, -race)	7m 13s
test (ubuntu-20.04, 1.23.x, rootless)	6m 54s
test (ubuntu-20.04, 1.23.x, -race)	1m 53s
test (ubuntu-20.04, 1.23.x)	1m 10s
test (ubuntu-20.04, 1.23.x, criu-dev)	1m 35s
test (ubuntu-24.04, 1.22.x, rootless, -race)	9m 18s
test (ubuntu-24.04, 1.22.x, rootless)	9m 11s
test (ubuntu-24.04, 1.22.x, -race)	1m 31s
test (ubuntu-24.04, 1.22.x)	50s
test (ubuntu-24.04, 1.23.x, rootless, -race)	9m 48s
test (ubuntu-24.04, 1.23.x, rootless)	9m 26s
test (ubuntu-24.04, 1.23.x, -race)	1m 41s
test (ubuntu-24.04, 1.23.x)	2m 3s
test (ubuntu-24.04, 1.23.x, criu-dev)	1m 56s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	11m 39s
test (actuated-arm64-6cpu-8gb, 1.23.x)	54s
all-done	0s
	1h 34m 7s

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