libct: speedup process.Env handling #7223

Job	Run time
cross-i386	1m 18s
test (ubuntu-20.04, 1.22.x, rootless, -race)	7m 14s
test (ubuntu-20.04, 1.22.x, rootless)	6m 53s
test (ubuntu-20.04, 1.22.x, -race)	7m 37s
test (ubuntu-20.04, 1.22.x)	7m 5s
test (ubuntu-20.04, 1.23.x, rootless, -race)	7m 3s
test (ubuntu-20.04, 1.23.x, rootless)	7m 1s
test (ubuntu-20.04, 1.23.x, -race)	7m 54s
test (ubuntu-20.04, 1.23.x)	7m 5s
test (ubuntu-20.04, 1.23.x, criu-dev)	7m 29s
test (ubuntu-24.04, 1.22.x, rootless, -race)	9m 42s
test (ubuntu-24.04, 1.22.x, rootless)	9m 22s
test (ubuntu-24.04, 1.22.x, -race)	7m 38s
test (ubuntu-24.04, 1.22.x)	7m 40s
test (ubuntu-24.04, 1.23.x, rootless, -race)	9m 47s
test (ubuntu-24.04, 1.23.x, rootless)	11m 6s
test (ubuntu-24.04, 1.23.x, -race)	8m 21s
test (ubuntu-24.04, 1.23.x)	7m 13s
test (ubuntu-24.04, 1.23.x, criu-dev)	8m 42s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	11m 37s
test (actuated-arm64-6cpu-8gb, 1.23.x)	7m 55s
all-done	0s
	2h 45m 42s

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