[SCons] Check instRoot instead of just prefix

Since the staging directory can be set when the prefix is the current
directory, the install should be able to continue in that case.

SConstruct

@@ -735,11 +735,6 @@ print()
 # *** Configure system-specific properties ***
 # ********************************************
 
-# Prevent setting prefix for Cantera installation to source directory
-if os.path.abspath(env['prefix']) == Dir('.').abspath:
-    print('ERROR: cannot install Cantera into source directory.')
-    exit(1)
-
 # Copy in external environment variables
 if env['env_vars'] == 'all':
     env['ENV'].update(os.environ)
@@ -1436,6 +1431,11 @@ if env['stage_dir']:
 else:
     instRoot = env['prefix']
 
+# Prevent setting Cantera installation path to source directory
+if os.path.abspath(instRoot) == Dir('.').abspath:
+    print('ERROR: cannot install Cantera into source directory.')
+    exit(1)
+
 if env['layout'] == 'debian':
     base = pjoin(os.getcwd(), 'debian')