Skip directories when showing hash values

[StykMartin]

Hi,

I encountered a case where the script failed due to the presence of .github directory. The rest of the action appears to handle directories appropriately, this part of the script doesn't.

This change is a short-term workaround. Ideally, the implementation should pass the artifacts explicitly rather than relying on iterdir().

Example

 Showing hash values of files to be uploaded:
/github/workspace/beaker_client-29.2-py3-none-any.whl

SHA256: 955b7a28b414818c450cdab6454a8dff99158e7b26ba2ab7f3bfb6809c948753
MD5: c98c5f772996268d337c229ebe96a94f
BLAKE2-256: b4e114f6f5a388f4e4db41b45bf2ff3c666242284a661c41b5725a08072cf990

/github/workspace/beaker_client-29.2-py3-none-any.whl.publish.attestation

Traceback (most recent call last):
SHA256: bb0b36aa46dbf9406ac41415249b6d5fe674d6c31d8ca2ab0dcfbcd037b41091
MD5: af075233cf503167a0d88027dc02af12
BLAKE2-256: 487b7f3dde89b965a7c22edddb3cf8f57d89f537e26c55f354fd60082d6bcc74

/github/workspace/.github

  File "/app/print-hash.py", line 17, in <module>
    content = file_object.read_bytes()
              ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/pathlib.py", line 1019, in read_bytes
    with self.open(mode='rb') as f:
         ^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/pathlib.py", line 1013, in open
    return io.open(self, mode, buffering, encoding, errors, newline)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
IsADirectoryError: [Errno 21] Is a directory: '/github/workspace/.github'