pdl: Set default-run for Cargo

This makes “cargo run” work again without arguments: when we added
src/bin/generate-canonical-tests.rs, “cargo run” started demanding
that we explicitly select a binary to run.

Test: cargo run
(cherry picked from https://android-review.googlesource.com/q/commit:c64db0408e1fc628d48da312c9bc133723f79354)
Merged-In: I787a4937d5f5be70d14f7ca221500ac8002bf7ae
Change-Id: I787a4937d5f5be70d14f7ca221500ac8002bf7ae