bump buildkit to v0.28.1

[DannyBrito]

What this PR does / why we need it:
dependabot trying to update to newer version of buildkit but fails as dalec is in lower version https://github.com/Azure/dalec-build-defs/pull/5646 as newever version seems to require new method SetImageBlobOption https://github.com/moby/buildkit/blame/v0.28.1/client/llb/source.go#L107

[Copilot]

Pull request overview

Updates Dalec’s Go module dependencies to support BuildKit v0.28.1 (needed by downstream consumers) and adds the missing llb.ConstraintsOpt hook required by the newer BuildKit LLB API.

Changes:

• Bump github.com/moby/buildkit to v0.28.1 (and refresh transitive deps in go.sum).
• Extend ConstraintsOptFunc to implement the new SetImageBlobOption(*llb.ImageBlobInfo) method.
• Update go.mod Go version directive.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 1 comment.

File	Description
helpers.go	Adds ConstraintsOptFunc.SetImageBlobOption to satisfy the updated BuildKit llb.ConstraintsOpt interface.
go.mod	Bumps BuildKit and related Go module versions; updates the go directive.
go.sum	Records updated dependency checksums after the module upgrades.

[Copilot]

The go directive should track the language version (typically 1.25 / 1.25.0), not a specific patch release. Using go 1.25.5 can also drift from CI which sets up go-version: '1.25' and may cause module parsing/tooling issues on environments that don’t recognize patch-level go directives. Consider reverting this to go 1.25.0 (or go 1.25) and, if you need to pin the toolchain patch, add a separate toolchain go1.25.5 line instead.

Suggested change

	go 1.25.5
	go 1.25.0
	toolchain go1.25.5

[cpuguy83]

Can we undo this change, by chance?

[DannyBrito]

I don't think so seems bump came from buildkit itself?
https://github.com/moby/buildkit/blob/v0.28.1/go.mod#L3