secret: Allocate bufer only once

gnome-keyring will always pass 64 bytes so we allocated only once. If
other implementations require:

 - less: Then we still allocate fewer times, and extra bytes do not harm
 - more: Then we still allocate fewer times than before

src/desktop/secret.rs

@@ -93,7 +93,7 @@ impl<'a> std::ops::Deref for Secret<'a> {
 /// It crates a UnixStream internally for receiving the secret.
 pub async fn retrieve() -> Result<Vec<u8>, Error> {
     let proxy = Secret::new().await?;
-    let mut buf = Vec::new();
+    let mut buf = Vec::with_capacity(64);
 
     #[cfg(feature = "tokio")]
     let mut x1 = {