flowey: vmm-tests must download alpine linux image

[mattkur]

We now have tests that run against alpine. The vmm-tests xflowey command wasn't downloading that image for the user. Add it.

[Copilot]

Pull request overview

Updates the local xflowey vmm-tests job so it downloads the Alpine Linux VHD artifacts needed by newly added Alpine-based VMM tests.

Changes:

• Add KnownTestArtifacts::Alpine323X64Vhd to the downloaded artifact set when the linux test selection flag is enabled (x86_64).
• Add KnownTestArtifacts::Alpine323Aarch64Vhd to the downloaded artifact set when the linux test selection flag is enabled (aarch64).

[Copilot]

Same selection-consistency issue as the x86_64 branch: Alpine aarch64 tests are named with alpine_3_23_aarch64 in the generated test names, so & !test(linux) won’t exclude them when linux is disabled. That can result in Alpine tests still running while this artifact isn’t downloaded.

Consider updating the filter (exclude test(alpine) when !linux) or adding a dedicated alpine flag used for both filtering and artifact downloads.

Suggested change

	if linux {
	artifacts.push(KnownTestArtifacts::Alpine323Aarch64Vhd);
	}
	artifacts.push(KnownTestArtifacts::Alpine323Aarch64Vhd);

[Copilot]

The linux selection flag currently only affects the nextest filter via & !test(linux) (and a few build toggles). Alpine-based tests generated by vmm_test_macros have names like *_uefi_*_alpine_3_23_* (they don’t contain the substring linux), so disabling linux won’t skip them. With this change, those Alpine artifacts also won’t be downloaded when linux is disabled, which can leave Alpine tests still selected but missing their required VHD.

To keep selections consistent, either (a) extend the filter logic so !linux also excludes Alpine tests (e.g., & !test(alpine) / & !test(alpine_3_23)), or (b) introduce an explicit alpine flag and use it for both filtering and artifact selection.