building: splash: replace lambda variable "l" with "x"

Under flake8 6.0.0, the "l" variable name in lambda function
seems to trigger E741. Rename the offending variable to "x".

PyInstaller/building/splash.py

@@ -448,7 +448,7 @@ def generate_script(self):
         if self.minify_script:
             # Remove any documentation, empty lines and unnecessary spaces
             script = '\n'.join(
-                line for line in map(lambda l: l.strip(), script.splitlines())
+                line for line in map(lambda x: x.strip(), script.splitlines())
                 if not line.startswith('#')  # documentation
                 and line  # empty lines
             )