cargo_fetch and rust PG "formal" bug

[RJVB (René Bertin)]

There's what one could call a formal bug in the post-extract sections of the cargo_fetch and rust PortGroups:

"${cargo.home}/config.toml is not re-written but appended to if cargo.crates nor cargo.crates_github are set.

This is a moot point for normal users, but for port maintainers who do not systematically do a port clean before attempting another build during port maintainance it leads to build failures as cargo apparently doesn't accept multiple [build] sections in that file.

Adding an else clause with

        # make sure config.toml doesn't contain any left-overs in this case too
        file delete -force "${cargo.home}/config.toml"

in local copies of the PGs fixed this issue for me.