Fix Windows CRLF Issue (#3455)

Cargo configs are injected as a literal string which, on Windows, can
end up with escaped CRLF line endings in `cargo-bazel.json` causing the
digests to be different on Linux/Windows. This PR parses the inner
string literal to prevent platform specific differences in the
`config.toml`. There might be a better way to do this, but adding a new
top level command to cargo-bazel to parse TOML into JSON felt wrong, and
adding a second binary seems even worse. I'm open to solving this
another way if ya'll have a better idea.