pdl: ensure we use ‘target/debug/pdl’ when running test with Cargo

The environment variable is documented here: https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-crates.

Test: m bluetooth_packetgen && rm -f out/host/linux-x86/bin/pdl && cargo test
Change-Id: I4c9a6bee25dd6d54a812513e60eeadaf394cd92f