Update cargo build instructions to use double colon syntax

[matthewhaynesonline]

Follow up of #3104 (comment)

Use cargo:: syntax for build instructions per https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script.

Notes

• This just simply replaces any of the existing cargo: build commands with cargo::.
• candle-examples/* passes $ cargo check
• candle-flash-attn and candle-kernels seem to rely on having NVIDIA drivers installed, so I couldn't run $ cargo check (I use a m4 macbook)

Questions

1. I see a cargo::info build command used (and passes check in candle-examples) but couldn't find reference to it in the docs. When I tried updating the bert binary example to use it instead of cargo::warn that seemed to fail. Should cargo::info be used instead of cargo::warn in the bert binary builder?

Compiling bert-single-file-binary-builder v0.9.1 (/Users/matt/Code/matthewhaynes/candle/candle-examples/bert_single_file_binary_builder)
error: invalid output in build script of `bert-single-file-binary-builder v0.9.1 (/Users/matt/Code/matthewhaynes/candle/candle-examples/bert_single_file_binary_builder)`: `cargo::info=All bert-single-file-binary-builder files already exist, skipping download`
Unknown key: `info`.
See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script for more information about build script outputs.

2. I don't think I can build / check candle-flash-attn and candle-kernels on my mac, but CI should ensure I didn't break anything build wise for CUDA - is that right?

[ivarflakstad]

1. I see a cargo::info ...

No documentation because it doesn't exist. Should be cargo::warning if you actually want to display a warning. For example I wouldn't show a warning for downloading files when that is the expected behaviour.

2. I don't think I can build

I don't think you will be able to break it by just using the correct amount of : in the build script instructions. When the PR is ready I can check on a cuda machine.