Decode messages from dotnet tool as UTF-8 (#1204)

If 'dotnet tool' outputs non-ASCII characters and the system locale is
not 'utf-8', then 'update_deps.py' can fail due to UnicodeDecodeError.
This commit addresses the above issue by explicitly setting 'utf-8' when
calling subprocess.Popen.

Closes #1203

PiperOrigin-RevId: 736025736

Author: yukawa

src/build_tools/update_deps.py

@@ -408,6 +408,7 @@ def exec_command(
   process = subprocess.Popen(
       args,
       cwd=cwd,
+      encoding='utf-8',
       shell=False,
       stdout=subprocess.PIPE,
       text=True,