Use os.getcwdb() instead of os.getcwd().encode() in util.cpio.CpioRead

Using os.getcwd() in combination with a subsequent .encode() is error
prone:

marcus@linux:~> mkdir illegal_utf-8_encoding_$'\xff'_dir
marcus@linux:~> cd illegal_utf-8_encoding_$'\xff'_dir/
marcus@linux:~/illegal_utf-8_encoding_ÿ_dir> python3
Python 3.8.6 (default, Nov 09 2020, 12:09:06) [GCC] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> os.getcwd().encode()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeEncodeError: 'utf-8' codec can't encode character '\udcff' in position 36: surrogates not allowed
>>>

Hence, use os.getcwdb(), which returns a bytes, instead of
os.getcwd().encode().

Fixes: commit 36f7b8ffe91ff759c938b1afce5f83be57b39671 ("Fix a
potential TypeError in CpioRead.copyin and CpioRead.copyin_file")

osc/util/cpio.py

@@ -182,7 +182,7 @@ class CpioRead:
         hdr = self._get_hdr(filename)
         if not hdr:
             raise CpioError(filename, '\'%s\' does not exist in archive' % filename)
-        dest = dest or os.getcwd().encode()
+        dest = dest or os.getcwdb()
         fn = new_fn or filename
         self._copyin_file(hdr, dest, fn)
 
@@ -191,7 +191,7 @@ class CpioRead:
         extracts the cpio archive to dest.
         If dest is None $PWD will be used.
         """
-        dest = dest or os.getcwd().encode()
+        dest = dest or os.getcwdb()
         for h in self.hdrs:
             self._copyin_file(h, dest, h.filename)