cargo: Iterate over a static copy of the keys

Solves:

```
Traceback (most recent call last):
  File "/home/bbhtt/Git/github/flatpak-builder-tools/cargo/flatpak-cargo-generator.py", line 503, in <module>
    main()
    ~~~~^^
  File "/home/bbhtt/Git/github/flatpak-builder-tools/cargo/flatpak-cargo-generator.py", line 490, in main
    generated_sources = asyncio.run(
        generate_sources(load_toml(args.cargo_lock), git_tarballs=args.git_tarballs)
    )
  File "/usr/lib64/python3.13/asyncio/runners.py", line 195, in run
    return runner.run(main)
           ~~~~~~~~~~^^^^^^
  File "/usr/lib64/python3.13/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/usr/lib64/python3.13/asyncio/base_events.py", line 725, in run_until_complete
    return future.result()
           ~~~~~~~~~~~~~^^
  File "/home/bbhtt/Git/github/flatpak-builder-tools/cargo/flatpak-cargo-generator.py", line 423, in generate_sources
    for pkg in await asyncio.gather(*pkg_coros):
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/bbhtt/Git/github/flatpak-builder-tools/cargo/flatpak-cargo-generator.py", line 381, in get_package_sources
    return await get_git_package_sources(package, git_repos)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/bbhtt/Git/github/flatpak-builder-tools/cargo/flatpak-cargo-generator.py", line 351, in get_git_package_sources
    "contents": tomlkit.dumps(git_pkg.normalized),
                              ^^^^^^^^^^^^^^^^^^
  File "/home/bbhtt/Git/github/flatpak-builder-tools/cargo/flatpak-cargo-generator.py", line 208, in normalized
    update_workspace_keys(package, self.workspace)
    ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/bbhtt/Git/github/flatpak-builder-tools/cargo/flatpak-cargo-generator.py", line 194, in update_workspace_keys
    update_workspace_keys(item, workspace_item)
    ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
  File "/home/bbhtt/Git/github/flatpak-builder-tools/cargo/flatpak-cargo-generator.py", line 158, in update_workspace_keys
    for key, item in pkg.items():
                     ~~~~~~~~~^^
  File "<frozen _collections_abc>", line 898, in __iter__
RuntimeError: dictionary keys changed during iteration
```

while generating at
pop-os/cosmic-comp@b28f92a

Author: bbhtt

cargo/flatpak-cargo-generator.py

@@ -155,7 +155,7 @@ def fetch_git_repo(git_url: str, commit: str) -> str:
 
 
 def update_workspace_keys(pkg: dict[str, Any], workspace: dict[str, Any]) -> None:
-    for key, item in pkg.items():
+    for key, item in list(pkg.items()):
         # There cannot be a 'workspace' key if the item is not a dict.
         if not isinstance(item, dict):
             continue