SCons: Fix running 'scons' without platform argument

The cache and progress logic assumed the 'env' to be defined,
but it is only when the selected platform is in the supported list.

Fixes #17497.

SConstruct

@@ -491,21 +491,18 @@ else:
     for x in platform_list:
         print("\t" + x)
     print("\nPlease run scons again with argument: platform=<string>")
-    sys.exit(255)
 
 
+# The following only makes sense when the env is defined, and assumes it is
+if 'env' in locals():
     screen = sys.stdout
+    # Progress reporting is not available in non-TTY environments since it
+    # messes with the output (for example, when writing to a file)
+    show_progress = (env['progress'] and sys.stdout.isatty())
     node_count = 0
     node_count_max = 0
     node_count_interval = 1
-if ('env' in locals()):
     node_count_fname = str(env.Dir('#')) + '/.scons_node_count'
-# Progress reporting is not available in non-TTY environments since it
-# messes with the output (for example, when writing to a file)
-if sys.stdout.isatty():
-    show_progress = env['progress']
-else:
-    show_progress = False
 
     import time, math
 
@@ -601,6 +598,7 @@ try:
             node_count_max = int(f.readline())
     except:
         pass
+
     cache_directory = os.environ.get("SCONS_CACHE")
     # Simple cache pruning, attached to SCons' progress callback. Trim the
     # cache directory to a size not larger than cache_limit.