check: add source and target_branch to metadata file

User might want to read the target_branch from concourse. This adds the target_branch field to the metadafile file in order to read it from Concourse side

For context I'm using this with the accross hack to do some Linux kernel build depending on the kernel version:

- name: merge-request-build
  public: true
  plan:
  - in_parallel:
    - get: merge-request
      params:
        skip_clone: true

  - load_var: merge-request-info 
    file: merge-request/.merge-request.json
    format: json

  - task: check_version
    config:
      platform: linux
      image_resource:
        type: registry-image
        source:
          repository: busybox
      inputs:
      - name: merge-request 
      outputs:
      - name: version
      run:
        path: sh
        args:
        - -exc
        - |
          cat merge-request/.merge-request.json
          if [ "((.:merge-request-info.target_branch))" == "5.15" ]; then
            echo ["5.15"] > version/5.15
            echo [] > version/6.12
            echo "It's a 5.15 kernel"
          else if [ "((.:merge-request-info.target_branch))" == "6.12" ]
            echo [] > version/5.15
            echo ["6.12"] > version/6.12
            echo "It's a 6.12 kernel"
          else
            echo [] > version/5.15
            echo [] > version/6.12
            echo "Not a known kernel"
          fi

  - load_var: is_515
    file: version/5.15
    format: json

  - across:
    - var: version
      values: ((.:is_515))
    do:
    - in_parallel:
      # HACK:
      # Put step call an implicit get step. Since skip_clone is not kept between 
      # get call, use the put to actually clone the repo. This way we can
      # dispatch to the correct "accros" quickly
      - put: merge-request
        params:
          resource_name: merge-request
          status: pending

    - task: build_5_15_kernel
      <...>

  - load_var: is_612
    file: version/6.12
    format: json

  - across:
    - var: version
      values: ((.:is_612))
    do:
    - in_parallel:
      # HACK:
      # Put step call an implicit get step. Since skip_clone is not kept between 
      # get call, use the put to actually clone the repo. This way we can
      # dispatch to the correct "accros" quickly
      - put: merge-request
        params:
          resource_name: merge-request
          status: pending

    - task: build_6_12_kernel
      <...>