fwupd: Check for changed file extension

Starting fwupd v1.9.16 the metadata compression was changed from xz to
zst. Check for the zst file instead of the xz file.

BUG=b:345572410
TEST=chromeos_unittests; manually deploy and check metadata is refreshed
correctly

Change-Id: I9de9d025cec4865233daa2126c5f27932d3ce04a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5617550


Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Jimmy Gong <jimmyxgong@chromium.org>
Commit-Queue: Rishabh Agrawal <rishabhagr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313061}