libct: speedup process.Env handling #7243

Job	Run time
cross-i386	1m 33s
test (ubuntu-20.04, 1.22.x, rootless, -race)	7m 22s
test (ubuntu-20.04, 1.22.x, rootless)	6m 48s
test (ubuntu-20.04, 1.22.x, -race)	7m 42s
test (ubuntu-20.04, 1.22.x)	7m 11s
test (ubuntu-20.04, 1.23.x, rootless, -race)	7m 1s
test (ubuntu-20.04, 1.23.x, rootless)	7m 2s
test (ubuntu-20.04, 1.23.x, -race)	7m 31s
test (ubuntu-20.04, 1.23.x)	7m 32s
test (ubuntu-20.04, 1.23.x, criu-dev)	7m 49s
test (ubuntu-24.04, 1.22.x, rootless, -race)	9m 36s
test (ubuntu-24.04, 1.22.x, rootless)	9m 39s
test (ubuntu-24.04, 1.22.x, -race)	7m 29s
test (ubuntu-24.04, 1.22.x)	6m 48s
test (ubuntu-24.04, 1.23.x, rootless, -race)	9m 53s
test (ubuntu-24.04, 1.23.x, rootless)	9m 21s
test (ubuntu-24.04, 1.23.x, -race)	7m 50s
test (ubuntu-24.04, 1.23.x)	7m 21s
test (ubuntu-24.04, 1.23.x, criu-dev)	7m 52s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	11m 38s
test (actuated-arm64-6cpu-8gb, 1.23.x)	7m 51s
all-done	1s
	2h 42m 50s

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