Use 'backslashreplace' not to crash on malformed UTF from subprocess

Giant commit
zephyrproject-rtos/hal_nxp@f9f0944bc2b4fce
"Update to SDK 2.14" added files with malformed UTF-8, more precisely
with the DEGREE SIGN (°) encoded in 8bit latin1/CP1252. Maybe others.

This crashes `west grep`.

Before this fix:

```
nxp$ west grep 'TEMPERATURE_CONV_FACTOR.*Will give'

Traceback (most recent call last):
  File ".local/bin/west", line 33, in <module>
    sys.exit(load_entry_point('west', 'console_scripts', 'west')())
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "west/src/west/app/main.py", line 1085, in main
    app.run(argv or sys.argv[1:])
  File "west/src/west/app/main.py", line 244, in run
    self.run_command(argv, early_args)
  File "west/src/west/app/main.py", line 503, in run_command
    self.run_builtin(args, unknown)
  File "west/src/west/app/main.py", line 611, in run_builtin
    self.cmd.run(args, unknown, self.topdir,
  File "west/src/west/commands.py", line 194, in run
    self.do_run(args, unknown)
  File "west/src/west/app/project.py", line 1765, in do_run
    completed_process = self.run_subprocess(
                        ^^^^^^^^^^^^^^^^^^^^
  File "west/src/west/commands.py", line 325, in run_subprocess
    return subprocess.run(args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/subprocess.py", line 550, in run
    stdout, stderr = process.communicate(input, timeout=timeout)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/subprocess.py", line 1209, in communicate
    stdout, stderr = self._communicate(input, endtime, timeout)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/subprocess.py", line 2146, in _communicate
    stdout = self._translate_newlines(stdout,
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/subprocess.py", line 1086, in _translate_newlines
    data = data.decode(encoding, errors)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xb0 in position 385: invalid start byte
```

After this fix, no crash and no interruption and:

mcux/mcux-sdk/middleware/issdk/sensors/fxpq3115_drv.h:#define FXPQ3115_TEMPERATURE_CONV_FACTOR (256) /* Will give \xb0C */
mcux/mcux-sdk/middleware/issdk/sensors/mpl3115_drv.h:#define MPL3115_TEMPERATURE_CONV_FACTOR (256) /* Will give \xb0C */

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

src/west/commands.py

@@ -321,7 +321,7 @@ def run_subprocess(self, args, **kwargs):
         the call at Verbosity.DBG_MORE level.'''
 
         self._log_subproc(args, **kwargs)
-        return subprocess.run(args, **kwargs)
+        return subprocess.run(args, errors='backslashreplace', **kwargs)
 
     def die_if_no_git(self):
         '''Abort if git is not installed on PATH.