Allow passing custom args to cargo in builds

[jyn514]

Right now, .cargo/config is stripped before the build, so there's no way to pass custom arguments to cargo during the build. It would be helpful to add cargo-args to the metadata in order to support flags like -Zbuild-std=std,alloc.

Originally posted by @Nemo157 in #1299 (comment)

[jyn514]

Mentoring instructions:

1. Add a new cargo_args field to Metadata, similar to rustdoc_args:

   docs.rs/crates/metadata/lib.rs

   Lines 129 to 131 in f6aef6d

   	/// List of command line arguments for `rustdoc`.
   	#[serde(default)]
   	rustdoc_args: Vec<String>,

2. Add it to cargo_args():

   docs.rs/crates/metadata/lib.rs

   Line 239 in f6aef6d

   pub fn cargo_args(&self, additional_args: &[String], rustdoc_args: &[String]) -> Vec<String> {

3. Document it on /about/builds:

   docs.rs/templates/core/Cargo.toml.example

   Lines 43 to 44 in f6aef6d

   	# Additional `RUSTDOCFLAGS` to set (default: [])
   	rustdoc-args = ["--example-rustdoc-arg"]