fix issue #358

[holg]

cargo leptos build looks for the server binary always in
target/debug
and then fails with:

Error: at ~/.cargo/git/checkouts/cargo-leptos-eb87bffcd2ca8f75/5dd5ec2/src/compile/server.rs:40:22

Caused by:
    0: at `~/.cargo/git/checkouts/cargo-leptos-eb87bffcd2ca8f75/5dd5ec2/src/service/site.rs:117:44
    1: Could not read "target/debug/test_leptos" at `~/.cargo/git/checkouts/cargo-leptos-eb87bffcd2ca8f75/5dd5ec2/src/ext/fs.rs:44:10
    2: No such file or directory (os error 2)

For convienience i added the current_platform = "0.2" to the dependencies
So in bin_package i added to test for the file to exist,
if not use the TARGET_TRIPLE before debug:

let mut test_file = file.join(profile.to_string())
            .join(name)
            .with_extension(file_ext);
        // Check if the file exists and if not, try to prepend target_triple
        // right now it mail fail to find target/debug/name
        // but the build is successful and in target/"target_triple"/debug/name
        // https://github.com/leptos-rs/cargo-leptos/issues/358
        if !test_file.exists(){
            test_file = Utf8PathBuf::from(format!(
                "target/{}/{}/{}",
                CURRENT_PLATFORM, profile.to_string(),test_file.file_name().unwrap()
            ));
        }
        test_file

[benwis]

@holg is this still an issue?