Keep complete V master toolchain in CI

[antono3]

Summary

• retain setup-v-action for the supported V 0.5.2 matrix entries
• build the advisory Linux V-master entry from an official shallow source checkout
• keep the v1_fallback executable required by current V-master tool commands such as v test
• preserve advisory failure handling for the moving master branch

Context

Current V master splits compatibility work into a separate v1_fallback binary. setup-v-action@v4 packages a reduced toolchain that omits it, causing the Vulkan v test ergonomic step to fail before testing this repository. The same direct-source setup has passed the complete OpenCL V-master test lane.

Verification

• workflow YAML parsed successfully with Ruby Psych
• git diff --check