build(deps): bump moby/buildkit from v0.25.1 to v0.26.2 in /ci

Bumps moby/buildkit from v0.25.1 to v0.26.2.

---
updated-dependencies:
- dependency-name: moby/buildkit
  dependency-version: v0.26.2
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

ci/Dockerfile-buildkit

@@ -1,3 +1,3 @@
 # We dont build from this dockerfile - we just parse the version, but storing
 # here means we get the dependabot updates
-FROM moby/buildkit:v0.25.1
+FROM moby/buildkit:v0.26.2