ci(vcpkg): Stabilize vcpkg binary caching (compiler-aware keys, explicit cache path)

[bobtista]

Somehow, the cache keys changed in the last two weeks, and an empty cache (189 bytes) was stored, causing all subsequent runs to "hit" an empty cache.

Identified several opportunities to improve it while troubleshooting:

• lukka/run-vcpkg@v11 sets a default temp directory, even when its caching is disabled. Explicitly overrode VCPKG_DEFAULT_BINARY_CACHE to ensure vcpkg always uses our persistent cache directory
• Include MSVC compiler version in the cache key to prevent cross-toolchain cache reuse
• Split cache restore and save steps for clearer control and observability
• Validate cache contents before saving to avoid writing empty caches
• Versioned the cache key (v2) to cleanly break from the poisoned cache

Build time is back down at 6 minutes again

[xezon]

Thanks for sending another fix. It is quite absurd that vcpkg cache breaks every month. All this new code you added, is all this totally necessary? Complexity looks outrageous and not human readable.

[bobtista]

Thanks for sending another fix. It is quite absurd that vcpkg cache breaks every month. All this new code you added, is all this totally necessary? Complexity looks outrageous and not human readable.

I removed some of the debugging lines. There's one potentially optional part that makes it so only one of the build jobs tries to save to the cache may be optional, but I think it avoids the errors when all 6 jobs try to write at the same time and only one succeeds.