Fix missing s! interpolation prefix in Python.lean error message

Without s!, "{pythonCmd}" prints as a literal string instead of
interpolating the variable. Same pattern as commit 729a4174b
(Fix #1244). Carried to main2 via merge #1311.

Author: MikaelMayer

StrataPythonTest/Util/Python.lean

@@ -105,7 +105,7 @@ def pythonCheckModule (pythonCmd : System.FilePath) (moduleName : String) : IO B
   | 1 =>
     return false
   | 255 =>
-    throw <| .userError "{pythonCmd} not found."
+    throw <| .userError s!"{pythonCmd} not found."
   | _ =>
     throw <| .userError
       s!"{pythonCmd} has unexpected exit code {exitCode}"