Support opting out of -Cdebuginfo=2 #190

Hi!

Codspeed Rust runner doesn't allow you to set your own debugnfo level and instead uses full debuginfo.
If you set RUSTFLAGS in CI, cargo-codspeed appends full debuginfo flags, and last flag setup wins in rustc.

We in Vortex build CUDA binaries and test them in Codspeed, and each binary's full debuginfo is around a gigabyte. Codspeed itself afaik doesn't read .debug_loc so debug = limited should be sufficient in all cases.

There's a way to hack own debuginfo level with CARGO_ENCODED_RUSTFLAGS, but this isn't a good solution.