Fix calling `proc_self_status()` more than once. #3769

Job	Run time
Rustfmt	3s
Check --no-default-features (nightly)	58s
Check nightly-only targets (nightly)	21s
Check (stable)	6m 9s
Check (nightly)	6m 24s
Check selected Tier 3 platforms (nightly)	3m 48s
Check (1.63)	11m 4s
Test (ubuntu)	44s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 4s
Test use-libc (ubuntu)	40s
Test (ubuntu-20.04)	42s
Test (i686-linux)	57s
Test use-libc (i686-linux)	1m 0s
Test use-libc (aarch64-linux)	1m 9s
Test (aarch64-linux)	1m 12s
Test use-libc (powerpc64le-linux)	1m 20s
Test (powerpc64le-linux)	1m 10s
Test use-libc (riscv64-linux)	1m 2s
Test (riscv64-linux)	1m 7s
Test (s390x-linux)	1m 8s
Test use-libc (arm-linux)	57s
Test (arm-linux)	1m 5s
Test (ubuntu-stable)	38s
Test (i686-linux-stable)	55s
Test (aarch64-linux-stable)	58s
Test (riscv64-linux-stable)	59s
Test (s390x-linux-stable)	58s
Test (powerpc64le-linux-stable)	1m 3s
Test (arm-linux-stable)	1m 3s
Test (ubuntu-1.63)	2m 45s
Test (i686-linux-1.63)	3m 1s
Test (aarch64-linux-1.63)	3m 5s
Test (riscv64-linux-1.63)	3m 0s
Test (s390x-linux-1.63)	3m 9s
Test (powerpc64le-linux-1.63)	3m 3s
Test (arm-linux-1.63)	3m 8s
Test (macos-latest)	18m 54s
Test (macos-11)	1m 7s
Test (windows)	55s
Test (windows-2019)	1m 1s
Test (musl)	38s
	1h 34m 24s

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