docs(pipelines/cargo): update inputs

Signed-off-by: Massimiliano Giovagnoli <massimiliano.giovagnoli@chainguard.dev>

pkg/build/pipelines/cargo/README.md

@@ -16,6 +16,7 @@ Compile an auditable rust binary with Cargo
 | modroot | false | Top directory of the rust package, this is where the target package lives. Before building, the cargo pipeline wil cd into this directory. Defaults to current working directory  | . |
 | opts | false | Options to pass to cargo build. Defaults to release  | --release |
 | output | false | Filename to use when writing the binary. The final install location inside the apk will be in prefix / install-dir / output  |  |
+| output-dir | false | Directory where the binaris will be placed after building. Defaults to target/release  | target/release |
 | prefix | false | Installation prefix. Defaults to usr  | usr |