ci: fix provider release through correct zipping

[elchead]

Context

The current Terraform provider release is broken because of wrong zipping.
Terraform expects the zip files to only contain the binary file, but our zip inflates to a directory. This leads to TF not finding the binary:
https://github.com/edgelesssys/constellation/actions/runs/7339043840/job/19982584425#step:18:62

See the relevant zipping line:

constellation/.github/workflows/draft-release.yml

Line 465 in 48343dd

zip -r "${folder_name}.zip" "${folder_name}"

You can check that this is done correctly for the edgelesstest, which works:

unzip terraform-provider-edgelesstest_1.0.0_linux_amd64.zip
Archive:  terraform-provider-edgelesstest_1.0.0_linux_amd64.zip
  inflating: CHANGELOG.md
  inflating: LICENSE.txt
  inflating: README.md
  inflating: terraform-provider-edgelesstest_1.0.0

Otherwise when downloading the provider release, the provider structure has an unexpected directory nesting, i.e.
the last directory of providers/registry.terraform.io/edgelesssys/constellation/2.14.0/darwin_arm64/terraform-provider-constellation_2.14.0_darwin_arm64 should not be there, and leads to TF not finding the binary.

Proposed change(s)

• only zip the binary and not the directory

Additional info

• starting a draft release

Checklist

• Update docs
• Add labels (e.g., for changelog category)
• Is PR title adequate for changelog?
• Link to Milestone

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