Cleanup Makefile and update versions

[bufdev]

• Update CI/Makefile to Go 1.25.0.
• Update buf to v1.56.0.
• Default to empty for two Makefile environment variables PKG and HOST_TARGET that were producing warnings.
• Remove extra whitespace in Makefile.
• make upgrade
• Re-run make generate, which was actually the original reason for this PR. Bump actions/checkout from 4 to 5 #21 failed because the build is non-deterministic due to not pinning the remote plugin versions in buf.gen.yaml and buf.gen.vt.yaml. I'd argue we keep them unpinned and always use latest, but an argument could go both ways.
• Rename buf.gen.vt.yaml to buf.vt.gen.yaml - .gen.yaml is the effective extension.

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf CI / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	⏩ skipped	✅ passed	✅ passed	Aug 18, 2025, 8:35 PM

[doriable]

Upgraded golangci-lint to 2.4.0 for go 1.25 support. Updated the config with the following, open to different decisions:

• disabled embeddedstructfieldcheck since our struct fields are deliberately structured for minimizing padding
• disabled noinlineerr since this is an established pattern that we use
• disabled wsl_v5 along with already disabled wsl
• added some exceptions for the use of exec.Command in hyperdump and hypertest tools

[mcy]

We can't merge this unless we run tests for both 1.24 still. Would you mind including both in the matrix? We can remove the 1.24 version later.