build/bake: use query format for git context

[crazy-max]

The build and bake workflows now enable query-style git context when computing the source context. The bake workflow also sets up Buildx before that helper is used so the context can be resolved in the same way as the build workflow.

Example: https://oci.dag.dev/?image=ghcr.io%2Fdocker%2Fgithub-builder-test%40sha256%3Ac460fef0460092229179846a3eab90b3fbc8860ede2929a13e5327651ceb439c

Provenance: https://oci.dag.dev/?blob=ghcr.io/docker/github-builder-test@sha256:7486a7663d84e055ac32eb3ba091200a38a2c4d4f7c48b8dd5deca5641cff8c8&mt=application%2Fvnd.in-toto%2Bjson&size=13965

  "predicate":{
    "buildDefinition":{
      "buildType":"https://github.com/moby/buildkit/blob/master/docs/attestations/slsa-definitions.md",
      "externalParameters":{
        "configSource":{
          "digest":{
            "sha1":"b7d6515f6f3eab38c6515ba2506b9d3f7c4b73af"
          },
          "path":"hello.Dockerfile",
          "uri":"https://github.com/docker/github-builder.git#refs/heads/git-query"
        },