fallback to `std::env::var("CARGO_PKG_NAME")`

[Umio-Yasuno]

Currently find-crate does not support workspace version, which causes an error in Manifest::crate_package(), which causes a compiler error.
cargo-i18n uses Manifest::crate_package() to get the crate name only, so I think that std::env::var("CARGO_PKG_NAME") can be used as a substitute.

Fix #97