Fix passphrase signing in GSC

[dimakuv]

After gramineproject/gramine#1197 is merged, there should be a corresponding PR in GSC repo, that would:

• Remove https://github.com/gramineproject/gsc/blob/master/sign.sh
• Remove passphrase arg from GSC itself:

  gsc/gsc.py

  Line 504 in a60a499

  sub_sign.add_argument('-p', '--passphrase', help='Passphrase for the signing key.')

• Fix around here:

  gsc/gsc.py

  Line 363 in a60a499

  shutil.copy(os.path.abspath('sign.sh'), tmp_build_sign_path)

• Fix around here:

  gsc/templates/Dockerfile.common.sign.template

  Line 8 in b7f0576

  RUN {% block path %}{% endblock %} /gramine/app_files/sign.sh \