fix: unsupported output in build script — heartwood

Installing with Cargo fails since cargo::rustc-env=RADICLE_VERSION=pre-release is invalid syntax.

         })
         .unwrap_or(0.to_string());
     println!("cargo::rustc-env=RADICLE_VERSION={version}");
     println!("cargo::rustc-env=GIT_COMMIT_TIME={commit_time}");
     println!("cargo::rustc-env=GIT_HEAD={hash}");
     println!("cargo:rustc-env=RADICLE_VERSION={version}");
     println!("cargo:rustc-env=GIT_COMMIT_TIME={commit_time}");
     println!("cargo:rustc-env=GIT_HEAD={hash}");
     Ok(())
 }