{Core} Always use UTF-8 for command metadata log file encoding

[jiasli]

Fix #17994
Adopt microsoft/knack#247

Context

On Windows with English as the system language (without UTC-8 enabled), the system encoding by default is cp1252 (Western Europe), and Python will use cp1252 as the file encoding by default.

Writing Unicode characters like 汉字 to log file results in error:

Traceback (most recent call last):
  File "C:\Users\xxx\AppData\Local\Programs\Python\Python38\lib\logging\__init__.py", line 1084, in emit
    stream.write(msg + self.terminator)
  File "C:\Users\xxx\AppData\Local\Programs\Python\Python38\lib\encodings\cp1252.py", line 19, in encode
    return codecs.charmap_encode(input,self.errors,encoding_table)[0]
UnicodeEncodeError: 'charmap' codec can't encode characters in position 87-88: character maps to <undefined>

Change

This PR forces command metadata log files to use UTF-8 so that az feedback works on non-UTF-8 systems as well.

[yonzhan]

Core

[yonzhan]

LGTM

[jiasli]

/azp run

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