ci: use gha cache for each ref

[crazy-max]

relates to #3

In previous attempt, we were caching each ref within the same build invocation using named contexts but seems parent targets are not able to solve the blobs. Since #8, we are now building a dedicated sandbox for each ref. Let's see if cache works as expected.

[crazy-max]

Nope still rebuild named context target in tests: https://github.com/moby/buildkit-bench/actions/runs/10524230865/job/29160539141#step:5:678 :cry:

  {
    "group": {
      "default": {
        "targets": [
          "tests"
        ]
      }
    },
    "target": {
      "buildkit-binaries": {
        "context": "https://github.com/moby/buildkit.git#v0.15.2",
        "dockerfile": "Dockerfile",
        "args": {
          "BUILDKIT_CONTEXT_KEEP_GIT_DIR": "1",
          "BUILDKIT_DEBUG": "1"
        },
        "cache-from": [
          "type=gha,scope=buildkit-9e14164a1099d3e41b58fc879cbdd6f2b2edb04e"
        ],
        "cache-to": [
          "type=gha,scope=buildkit-9e14164a1099d3e41b58fc879cbdd6f2b2edb04e"
        ],
        "target": "binaries",
        "output": [
          "type=cacheonly"
        ]
      },
      "tests": {
        "context": ".",
        "contexts": {
          "buildkit-binaries": "target:buildkit-binaries"
        },
        "dockerfile": "Dockerfile",
        "args": {
          "BUILDKIT_REF": "v0.15.2"
        },
        "cache-from": [
          "type=gha,scope=buildkit-9e14164a1099d3e41b58fc879cbdd6f2b2edb04e"
        ],
        "cache-to": [
          "type=gha,scope=buildkit-9e14164a1099d3e41b58fc879cbdd6f2b2edb04e"
        ],
        "target": "tests",
        "output": [
          "type=docker,name=buildkit-bench"
        ]
      }
    }
  }