der: fix CI config for path to `der_derive` (#1460) #1241

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	30s
build (1.75.0, wasm32-unknown-unknown)	22s
build (stable, thumbv7em-none-eabi)	1s
test (1.75.0)	36s
build (stable, wasm32-unknown-unknown)	1s
test (stable)	21s
minimal-versions	0s
	1m 51s

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