der: fix CI config for path to `der_derive` (#1460) #671

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	31s
build (1.75.0, wasm32-unknown-unknown)	32s
build (stable, thumbv7em-none-eabi)	29s
build (stable, wasm32-unknown-unknown)	27s
minimal-versions	0s
test (1.75.0)	42s
test (stable)	30s
	3m 11s

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