deps: update module github.com/moby/buildkit to v0.21.1 (#946)

This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [github.com/moby/buildkit](https://redirect.github.com/moby/buildkit)
| `v0.21.0` -> `v0.21.1` |
[![age](https://developer.mend.io/api/mc/badges/age/go/github.com%2fmoby%2fbuildkit/v0.21.1?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![adoption](https://developer.mend.io/api/mc/badges/adoption/go/github.com%2fmoby%2fbuildkit/v0.21.1?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![passing](https://developer.mend.io/api/mc/badges/compatibility/go/github.com%2fmoby%2fbuildkit/v0.21.0/v0.21.1?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![confidence](https://developer.mend.io/api/mc/badges/confidence/go/github.com%2fmoby%2fbuildkit/v0.21.0/v0.21.1?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|

---

### Release Notes

<details>
<summary>moby/buildkit (github.com/moby/buildkit)</summary>

###
[`v0.21.1`](https://redirect.github.com/moby/buildkit/compare/v0.21.0...v0.21.1)

[Compare
Source](https://redirect.github.com/moby/buildkit/compare/v0.21.0...v0.21.1)

</details>

---

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---

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go.mod

@@ -11,7 +11,7 @@ require (
 	github.com/hashicorp/nomad/api v0.0.0-20250425125325-3e688cf9282f
 	github.com/hetznercloud/hcloud-go/v2 v2.21.0
 	github.com/kubernetes-csi/csi-test/v5 v5.3.1
-	github.com/moby/buildkit v0.21.0
+	github.com/moby/buildkit v0.21.1
 	github.com/prometheus/client_golang v1.22.0
 	github.com/stretchr/testify v1.10.0
 	golang.org/x/sys v0.32.0

go.sum

@@ -159,6 +159,8 @@ github.com/moby/buildkit v0.20.2 h1:qIeR47eQ1tzI1rwz0on3Xx2enRw/1CKjFhoONVcTlMA=
 github.com/moby/buildkit v0.20.2/go.mod h1:DhaF82FjwOElTftl0JUAJpH/SUIUx4UvcFncLeOtlDI=
 github.com/moby/buildkit v0.21.0 h1:+z4vVqgt0spLrOSxi4DLedRbIh2gbNVlZ5q4rsnNp60=
 github.com/moby/buildkit v0.21.0/go.mod h1:mBq0D44uCyz2PdX8T/qym5LBbkBO3GGv0wqgX9ABYYw=
+github.com/moby/buildkit v0.21.1 h1:wTjVLfirh7skZt9piaIlNo8WdiPjza1CDl2EArDV9bA=
+github.com/moby/buildkit v0.21.1/go.mod h1:mBq0D44uCyz2PdX8T/qym5LBbkBO3GGv0wqgX9ABYYw=
 github.com/moby/sys/mountinfo v0.7.2 h1:1shs6aH5s4o5H2zQLn796ADW1wMrIwHsyJ2v9KouLrg=
 github.com/moby/sys/mountinfo v0.7.2/go.mod h1:1YOa8w8Ih7uW0wALDUgT1dTTSBrZ+HiBLGws92L2RU4=
 github.com/moby/sys/userns v0.1.0 h1:tVLXkFOxVu9A64/yh59slHVv9ahO9UIev4JZusOLG/g=