update rust toolchain

It comes with a bunch of new lints (yay!) so I fixed them all so CI will
keep working.

Also apparently something about linking changed because I had to change
the checks for deciding the linker flags for static x86_64 builds to
keep working.

src/database/key_value/users.rs

@@ -146,10 +146,9 @@ impl service::users::Data for KeyValueDatabase {
         self.userid_avatarurl
             .get(user_id.as_bytes())?
             .map(|bytes| {
-                let s = utils::string_from_bytes(&bytes)
-                    .map_err(|_| Error::bad_database("Avatar URL in db is invalid."))?;
-                s.try_into()
+                utils::string_from_bytes(&bytes)
                     .map_err(|_| Error::bad_database("Avatar URL in db is invalid."))
+                    .map(Into::into)
             })
             .transpose()
     }