test(agc): word the lavapipe skips of two tests as device limitations

[adriadam10]

What

Main's linux job fails check_skips since #4451 and #5520 (run 38556304009): agc_driver_dithered_alpha_to_coverage and agc_shadowed_key_stores skip on lavapipe with messages the device-limitation pattern of tools/check_skips.py doesn't match. Both skips are device limitations (no eight-sample color attachments; no watched host import under a unit shadow), so their messages now use the pattern's wording (the device lacks, the device imports no).

Tested

Linux: both new messages match DEVICE_LIMITATION; no other reference to the old texts.

Checklist

• Based on current main; no other open PR implements the same functions
• One topic per PR; follow-ups go in a new PR
• No comments in code except technical debt
• Unimplemented paths throw (NotImplemented_nid_no_patch); silent stubs are listed under Silent stubs in their library's technical debt file
• No notes, investigation .md files or images added to the repository (attach them to this PR)
• New third-party code is a submodule built from source
• Generic behaviour, not specific to one title
• Depends on: none
• Unblocks: none
• GPUs tested: n/a
• AI-assisted: yes