libct: speedup process.Env handling #7244

Job	Run time
cross-i386	42s
test (ubuntu-20.04, 1.22.x, rootless, -race)	7m 2s
test (ubuntu-20.04, 1.22.x, rootless)	6m 54s
test (ubuntu-20.04, 1.22.x, -race)	7m 45s
test (ubuntu-20.04, 1.22.x)	7m 6s
test (ubuntu-20.04, 1.23.x, rootless, -race)	6m 59s
test (ubuntu-20.04, 1.23.x, rootless)	7m 12s
test (ubuntu-20.04, 1.23.x, -race)	7m 29s
test (ubuntu-20.04, 1.23.x)	7m 13s
test (ubuntu-20.04, 1.23.x, criu-dev)	7m 43s
test (ubuntu-24.04, 1.22.x, rootless, -race)	10m 39s
test (ubuntu-24.04, 1.22.x, rootless)	9m 43s
test (ubuntu-24.04, 1.22.x, -race)	7m 35s
test (ubuntu-24.04, 1.22.x)	6m 49s
test (ubuntu-24.04, 1.23.x, rootless, -race)	10m 18s
test (ubuntu-24.04, 1.23.x, rootless)	10m 43s
test (ubuntu-24.04, 1.23.x, -race)	7m 54s
test (ubuntu-24.04, 1.23.x)	7m 22s
test (ubuntu-24.04, 1.23.x, criu-dev)	7m 45s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	11m 42s
test (actuated-arm64-6cpu-8gb, 1.23.x)	7m 55s
all-done	1s
	2h 44m 31s

Provide feedback