ci: update buildx to 0.19.0 and buildkit to 0.18.0

[crazy-max]

needs #514

[crazy-max]

Tests fail with Buildx 0.19.0: https://github.com/docker/actions-toolkit/actions/runs/12047687153/job/33590720877?pr=507#step:3:923

#23 19.93   ● getDefinition › given ["/src/__tests__/.fixtures/bake-01.hcl"]
#23 19.93 
#23 19.93     expect(received).toEqual(expected) // deep equality
#23 19.93 
#23 19.93     - Expected  - 3
#23 19.93     + Received  + 9
#23 19.93 
#23 19.93     @@ -20,11 +20,13 @@
#23 19.93               "GO_VERSION": "1.20",
#23 19.93             },
#23 19.93             "context": ".",
#23 19.93             "dockerfile": "./hack/dockerfiles/lint.Dockerfile",
#23 19.93             "output": Array [
#23 19.93     -         "type=cacheonly",
#23 19.93     +         Object {
#23 19.93     +           "type": "cacheonly",
#23 19.93     +         },
#23 19.93             ],
#23 19.93           },
#23 19.93           "validate-docs": Object {
#23 19.93             "args": Object {
#23 19.93               "BUILDKIT_CONTEXT_KEEP_GIT_DIR": "1",
#23 19.93     @@ -33,11 +35,13 @@
#23 19.93               "GO_VERSION": "1.20",
#23 19.93             },
#23 19.93             "context": ".",
#23 19.93             "dockerfile": "./hack/dockerfiles/docs.Dockerfile",
#23 19.93             "output": Array [
#23 19.93     -         "type=cacheonly",
#23 19.93     +         Object {
#23 19.93     +           "type": "cacheonly",
#23 19.93     +         },
#23 19.93             ],
#23 19.93             "target": "validate",
#23 19.93           },

Relates to #503 (cc @jsternberg)