Support custom build commands

[mainrs]

This can be useful if one wants to use a build command proxy like cargo-auditable. The binary includes a dependency tree and makes it possible to audit a released binary.

Instead of running a normal cargo build --release, one would run cargo auditable build --release.

[psionic-k]

This can be done with a builder override for the very last crate, but I should also add support for the mkcrate builder