How do I change the default rustc / Cargo linker?

Thanks to @Jmb comment, I found a solution. Turns out that the default linker that rustc uses is actually cc (which makes sense – it supplies all the needed defaults to compile/link C code, which also work for Rust). We can pass an argument to cc to make it link with lld:

[target.x86_64-unknown-linux-gnu]
rustflags = [
    "-C", "link-arg=-fuse-ld=lld",
]

Now cargo build links with lld.