Fix linux packaging step for workflow_call events

[mdeggies]

This PR should fix the linux packaging build failure happening within the nightly-release github actions workflow. A recent failure is available here: https://github.com/hashicorp/packer/runs/5341017655?check_suite_focus=true.

When the build workflow is called from the nightly-release workflow, ${{ github.event.repository.name }} evaluates to nothing because this value isn't available on the workflow_call event. This value should evaluate to packer, so we're just reusing the env var PKG_NAME to set that.

---

More detail:

If we look at a recent run from the build job triggered on a merge to master, https://github.com/hashicorp/packer/runs/5341305084?check_suite_focus=true, this shows the inputs to the linux packaging action:

Run hashicorp/actions-packaging-linux@v1
  with:
    name: packer
    description: HashiCorp Packer - A tool for creating identical machine images for multiple platforms from a single source configuration
    arch: arm
    version: 1.7.11-dev
    maintainer: HashiCorp
    homepage: https://www.packer.io/docs
    license: MPL-[2](https://github.com/hashicorp/packer/runs/5341305084?check_suite_focus=true#step:7:2).0
    binary: dist/packer
    deb_depends: openssl
    rpm_depends: openssl
    config_dir: .release/linux/package/
    preinstall: .release/linux/preinst
    postremove: .release/linux/postrm

On the failing job, the inputs look like this:

Run hashicorp/actions-packaging-linux@v1
  with:
    description: HashiCorp Packer - A tool for creating identical machine images for multiple platforms from a single source configuration
    arch: 386
    version: 1.7.11-dev
    maintainer: HashiCorp
    homepage: https://www.packer.io/docs
    license: MPL-[2](https://github.com/hashicorp/packer/runs/5341017655?check_suite_focus=true#step:7:2).0
    binary: dist/packer
    deb_depends: openssl
    rpm_depends: openssl
    config_dir: .release/linux/package/
    preinstall: .release/linux/preinst
    postremove: .release/linux/postrm

Notice the missing (required) name field.

[nywilken]

LGTM - thanks for the detailed description.

[github-actions]

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