der

der: fixup windows `write_secret_file` helper #2770

Job	Run time
build (1.71.0, thumbv7em-none-eabi)	1m 40s
build (1.71.0, wasm32-unknown-unknown)	1m 29s
build (stable, thumbv7em-none-eabi)	1m 28s
derive (1.71.0)	25s
build (stable, wasm32-unknown-unknown)	1m 20s
derive (stable)	23s
test (i686-unknown-linux-gnu, ubuntu-latest, 1.71.0, sudo apt update && sudo apt install gcc-mult...	11m 0s
test (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mult...	10m 29s
test (x86_64-unknown-linux-gnu, ubuntu-latest, 1.71.0)	10m 33s
test (x86_64-unknown-linux-gnu, ubuntu-latest, stable)	10m 7s
minimal-versions	0s
	48m 54s

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