Make `cargo run` use `zebrad` by default

When `cargo run` is run in the workspace directory, it can see two
executables:
- `zebrad`
- `zebra_checkpoints`

Adding `default-run = "zebrad"` to `zebrad/Cargo.toml` makes the
workspace run `zebrad` by default. (Even though it's redundant for the
`zebrad` crate itself.)

zebrad/Cargo.toml

@@ -5,6 +5,9 @@ license = "MIT OR Apache-2.0"
 version = "1.0.0-alpha.0"
 edition = "2018"
 repository = "https://github.com/ZcashFoundation/zebra"
+# make `cargo run` use `zebrad` by default
+# when run in the workspace directory
+default-run = "zebrad"
 
 [dependencies]
 zebra-chain = { path = "../zebra-chain" }