chore: install just in the devcontainer for Linux development (#1375)

Apparently `just` was added to `apt` in Ubuntu 24, so this required
updating the Ubuntu version in the `Dockerfile` to make it so we could
simply `apt install just`.

Though then that caused a conflict with the custom `dev` user we were
using, though the end result seems simpler since now we just use the
default `ubuntu` user provided by Ubuntu 24.

.devcontainer/devcontainer.json

@@ -15,15 +15,13 @@
     "CARGO_TARGET_DIR": "${containerWorkspaceFolder}/codex-rs/target-arm64"
   },
 
-  "remoteUser": "dev",
+  "remoteUser": "ubuntu",
   "customizations": {
     "vscode": {
       "settings": {
-          "terminal.integrated.defaultProfile.linux": "bash"
+        "terminal.integrated.defaultProfile.linux": "bash"
       },
-      "extensions": [
-          "rust-lang.rust-analyzer"
-      ],
+      "extensions": ["rust-lang.rust-analyzer"]
     }
   }
 }