Do not publish CAPI crate

There's no advantage of having this crate published, it must be built
using `cargo-c`, which can't fetch it over the network (yet?). It also
doesn't make sense depending on this crate; if you do, you should rather
use the main Rust crate `pineappl`.

Author: cschwan

pineappl_capi/Cargo.toml

@@ -6,6 +6,7 @@ authors = [
 description = "C language interface to PineAPPL"
 name = "pineappl_capi"
 readme = "README.md"
+publish = false
 
 categories.workspace = true
 edition.workspace = true