chore: update moby/buildkit to v0.20.1 (#519)

**Description:**  
This PR updates the `moby/buildkit` dependency to `v0.20.1` in order to
remain compatible with recent changes required by GitHub Actions (GHA)
caching.

During the upgrade, several upstream behavioral changes required forking
and restoring legacy logic to maintain compatibility:

- Introduced `containsGithubToken` to replicate logic that was
previously in `buildflags.ParseCacheEntry`, which is now removed. This
check ensures we return `nil` instead of a zero-value object.
- Forked the previous implementation of `ParseExports`, as its upstream
logic changed significantly. This ensures short-term compatibility for
release before the GHA deprecation deadline.
- A follow-up issue will be created to improve this forked logic and
align more closely with upstream behavior.

🚨 **Fixes critical issue:**  
This change addresses a high-priority issue where GitHub Actions cache
will stop working due to the upcoming deprecation of the legacy caching
service (effective **April 15, 2025**).

Recent build failures showed errors like:
> *"This legacy service is shutting down, effective April 15, 2025.
Migrate to the new service ASAP. For more information:
https://gh.io/gha-cache-sunset"*

Root cause was identified in #515, where using `buildx >= 0.21.0` is
required.
This repo was previously using `buildx 0.18.0`.

Closes: #515

sdk/go/dockerbuild/go.mod

@@ -26,6 +26,7 @@ require (
 	github.com/cheggaaa/pb v1.0.29 // indirect
 	github.com/cloudflare/circl v1.6.0 // indirect
 	github.com/cyphar/filepath-securejoin v0.4.1 // indirect
+	github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc // indirect
 	github.com/djherbis/times v1.6.0 // indirect
 	github.com/emirpasic/gods v1.18.1 // indirect
 	github.com/erikgeiser/coninput v0.0.0-20211004153227-1c3628e74d0f // indirect
@@ -60,6 +61,7 @@ require (
 	github.com/pjbgf/sha1cd v0.3.2 // indirect
 	github.com/pkg/errors v0.9.1 // indirect
 	github.com/pkg/term v1.1.0 // indirect
+	github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2 // indirect
 	github.com/pulumi/appdash v0.0.0-20231130102222-75f619a67231 // indirect
 	github.com/pulumi/esc v0.13.0 // indirect
 	github.com/rivo/uniseg v0.4.7 // indirect