apt: repositories: remove unnecessary if-let in iterator

Fixes the manual_flatten clippy lint:

```
warning: unnecessary `if let` since only the `Some` variant of the iterator element is used
  --> proxmox-apt/src/repositories/mod.rs:40:5
   |
40 |       for digest in digests.values() {
   |       ^             ---------------- help: try: `digests.values().copied().flatten()`
   |  _____|
   | |
41 | |         if let Some(digest) = digest {
42 | |             common_raw.extend_from_slice(&digest[..]);
43 | |         }
44 | |     }
   | |_____^
   |
help: ...and remove the `if let` statement in the for loop
  --> proxmox-apt/src/repositories/mod.rs:41:9
   |
41 | /         if let Some(digest) = digest {
42 | |             common_raw.extend_from_slice(&digest[..]);
43 | |         }
   | |_________^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_flatten
   = note: `#[warn(clippy::manual_flatten)]` on by default
```

Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
FG: use `into_values().flatten()` instead of `values().copied().flatten()
Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>

proxmox-apt/src/repositories/mod.rs

@@ -37,10 +37,8 @@ fn common_digest(files: &[APTRepositoryFile]) -> ConfigDigest {
     }
 
     let mut common_raw = Vec::<u8>::with_capacity(digests.len() * 32);
-    for digest in digests.values() {
-        if let Some(digest) = digest {
-            common_raw.extend_from_slice(&digest[..]);
-        }
+    for digest in digests.into_values().flatten() {
+        common_raw.extend_from_slice(&digest[..]);
     }
 
     ConfigDigest::from_slice(&common_raw[..])