One more unixy fix.

test cases/failing build/5 failed pickled/false.py

@@ -0,0 +1,4 @@
+#!/usr/bin/env python3
+
+import sys
+sys.exit(1)

test cases/failing build/5 failed pickled/meson.build

@@ -1,7 +1,7 @@
 project('failed pickled command')
 
 custom_target('failure',
-  command: ['false', '\n'],
+  command: [find_program('false.py'), '\n'],
   output: 'output.txt',
   build_by_default: true,
 )