scripts: west_commands: correct f-string in board mismatch error message

The board mismatch error message in `scripts/west_commands/build.py` was
partially missing f-string formatting. This caused `{self.args.board}`
to be printed literally instead of being replaced with the actual board
value.

Fix the f-string.

Signed-off-by: Miguel Gazquez <miguel.gazquez@bootlin.com>