ci: add weekly lxd tests

[lengau]

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• Have you signed the CLA?
• Have you successfully run make lint && make test?

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This has us running our LXD instance tests on a matrix of Ubuntu/lxd/Python versions each week.

Example run: https://github.com/canonical/craft-providers/actions/runs/19977333039

This should catch errors such as #856 earlier.

[Copilot]

Pull request overview

This PR adds a new weekly CI workflow job to test LXD functionality across multiple Ubuntu versions, LXD channels, and Python versions. The job runs tests specifically marked with 'lxd_instance' to validate LXD provider compatibility across different environment combinations.

• Adds a new lxd job to the weekly tests workflow
• Tests multiple Ubuntu versions (22.04, 24.04) and architectures (x86_64, ARM)
• Validates compatibility with various LXD channels (4.0, 5.0, 5.21, and latest)

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[bepri]

All of the current failures in the weekly spreads (that I found) seem to be this:

 tests/integration/lxd/test_lxc.py:179: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <craft_providers.lxd.lxc.LXC object at 0x7fe912fa4730>

    def image_copy(
        self,
        *,
        image: str,
        image_remote: str,
        alias: str | None = None,
        project: str = "default",
        remote: str = "local",
    ) -> None:
        """Copy image.
    
        :param instance_name: Optional instance name.
        :param alias: New alias to add to image.
        :param image: Image to copy.
        :param project: Name of LXD project.
        :param remote: Name of LXD remote.
    
        :raises LXDError: on unexpected error.
        """
        command = ["image", "copy", f"{image_remote}:{image}", f"{remote}:"]
    
        if alias is not None:
            command.append(f"--alias={alias}")
    
        try:
            self._run_lxc(command, capture_output=True, project=project)
        except subprocess.CalledProcessError as error:
>           raise LXDError(
                brief=f"Failed to copy image {image!r}.",
                details=errors.details_from_called_process_error(error),
            ) from error
E           craft_providers.lxd.errors.LXDError: Failed to copy image '22.04'.
E           * Command that failed: 'lxc --project craft-providers-test-session image copy ubuntu:22.04 local: --alias=test-2204'

Is this a network error bubbling up strangely, or are we actually referencing a nonexistent image here?

[lengau]

@bepri it's a network issue. It succeeded in some environments but not others.