securesystemslib.gpg cannot handle absolute paths on Windows

[lukpueh]

When passing an absolute path as homedir argument to create_signature or export_pubkey in securesystemslib.gpg.functions on Windows, gpg seems to treat it as relative path and prepends it with cwd:

Python 3.11.2 (tags/v3.11.2:878ead1, Feb  7 2023, 16:38:35) [MSC v.1934 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> import securesystemslib.gpg.functions as gpg
>>> gnupg_home = os.path.realpath("rsa")
>>> gnupg_home
'D:\\a\\securesystemslib\\securesystemslib\\tests\\gpg_keyrings\\rsa'
>>> gpg.create_signature(b"data", homedir=gnupg_home)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "D:\a\securesystemslib\securesystemslib\securesystemslib\gpg\functions.py", line 141, in create_signature
    raise CommandError(
securesystemslib.gpg.exceptions.CommandError: Command '['gpg', '--detach-sign', '--digest-algo', 'SHA256', '--homedir', 'D:/a/securesystemslib/securesystemslib/tests/gpg_keyrings/rsa']' returned non-zero exit status '2', stderr was:
gpg: keyblock resource '/d/a/securesystemslib/securesystemslib/tests/gpg_keyrings/D:/a/securesystemslib/securesystemslib/tests/gpg_keyrings/rsa/pubring.kbx': No such file or directory
gpg: failed to create temporary file '/d/a/securesystemslib/securesystemslib/tests/gpg_keyrings/D:/a/securesystemslib/securesystemslib/tests/gpg_keyrings/rsa/.#lk0x0000000a0000b3a0.fv-az89-415.1589': No such file or directory
gpg: can't connect to the agent: No such file or directory
gpg: no default secret key: No secret key
gpg: signing failed: No secret key
.

It works as expected, when passing a relative path:

# ... continued from above
>>> gpg.create_signature(b"data", homedir="rsa")
{'keyid': 'c5a0abe6ec19d0d65f85e2c39be9df5131d924e9', 'other_headers': '040001...', 'signature': 'b2c4f4...'}