Fix Snap build

Rustc & cargo are now installed using the script provided by
rust-lang.org. Their versions were bumped to 1.68.2. The (outdated)
versions bundled to core22 are not used anymore.

(Solution copied from https://forum.snapcraft.io/t/correct-snapcraft-syntax-for-rust-based-snap-on-core22/30947/5)

snap/snapcraft.yaml

@@ -10,12 +10,12 @@ confinement: strict # use 'strict' once you have the right plugs and slots
 compression: lzo
 
 parts:
-  user-part:
-    source: .
-    plugin: rust
-    build-packages: [cargo, rustc]
+  rust-deps:
+    plugin: nil
+    override-pull: curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --profile minimal --default-toolchain 1.68.2
   czkawka:
     plugin: rust
+    after: [ rust-deps ]
     source: https://github.com/qarmin/czkawka.git
     build-packages:
       - curl