[test] lit: Delay setting up %env- substitutions until we have target_env_prefix fully configured

This resolves variables not properly propagating through remote-run invocations during testing.

Author: lorentey

test/lit.cfg

@@ -1954,12 +1954,6 @@ def configure_remote_run():
         remote_tmp_dir, 'bin', swift_reflection_test_name)
     config.available_features.add('remote_run')
 
-# Different OS's require different prefixes for the environment variables to be
-# propagated to the calling contexts.
-# In order to make tests OS-agnostic, names of environment variables should be
-# prefixed with `%env-`, which is then expanded to the appropriate prefix.
-config.substitutions.append(('%env-', config.target_env_prefix))
-
 config.substitutions.append(("%target-sdk-name", config.target_sdk_name))
 
 simulator_sdks = [
@@ -2286,6 +2280,12 @@ if platform.system() != 'Darwin' or swift_test_mode == 'optimize_none_with_impli
 # When changing substitutions, update docs/Testing.md.
 #
 
+# Different OS's require different prefixes for the environment variables to be
+# propagated to the calling contexts.
+# In order to make tests OS-agnostic, names of environment variables should be
+# prefixed with `%env-`, which is then expanded to the appropriate prefix.
+config.substitutions.append(('%env-', config.target_env_prefix))
+
 config.substitutions.append(('%target-clangxx', '%s -std=c++11' % config.target_clang))
 config.substitutions.append(('%target-swiftxx-frontend', '%s -enable-experimental-cxx-interop' % config.target_swift_frontend))