Fix `_atomic_save` staging local checkpoints through `$TMPDIR` #30661

Job	Run time
build-packages / build-packages (pytorch)	24s
build-packages / build-packages (lightning)	26s
build-packages / build-packages (fabric)	25s
build-packages / merge-artifacts	8s
install-pkg (ubuntu-22.04, lightning, 3.11)	1m 35s
install-pkg (ubuntu-22.04, pytorch, 3.11)	1m 45s
install-pkg (macOS-14, fabric, 3.11)	35s
install-pkg (ubuntu-22.04, notset, 3.11)	1m 37s
install-pkg (ubuntu-22.04, fabric, 3.11)	1m 40s
install-pkg (ubuntu-22.04, pytorch, 3.10)	1m 29s
install-pkg (macOS-14, lightning, 3.10)	52s
install-pkg (ubuntu-22.04, notset, 3.10)	1m 36s
install-pkg (ubuntu-22.04, lightning, 3.10)	1m 34s
install-pkg (macOS-14, fabric, 3.10)	50s
install-pkg (macOS-14, pytorch, 3.10)	52s
install-pkg (ubuntu-22.04, fabric, 3.10)	1m 15s
install-pkg (macOS-14, lightning, 3.11)	41s
install-pkg (windows-2022, pytorch, 3.10)	1m 56s
install-pkg (windows-2022, notset, 3.11)	1m 56s
install-pkg (windows-2022, notset, 3.10)	1m 51s
install-pkg (macOS-14, notset, 3.10)	49s
install-pkg (macOS-14, pytorch, 3.11)	40s
install-pkg (windows-2022, lightning, 3.10)	2m 0s
install-pkg (windows-2022, fabric, 3.11)	1m 44s
install-pkg (windows-2022, fabric, 3.10)	2m 6s
install-pkg (macOS-14, notset, 3.11)	34s
install-pkg (windows-2022, lightning, 3.11)	1m 50s
install-pkg (windows-2022, pytorch, 3.11)	2m 3s
install-pkg-guardian	3s
	35m 16s

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