Flush build step log messages in build_script_invocation.py (swiftlang#77324)

This ensures that log messages are printed as soon as a corresponding build step starts, instead of printing these messages in the end out of order.

For example, before for SwiftPM

```
--- Cleaning swiftpm ---
--- Building swiftpm ---
--- Running tests for swiftpm ---
--- Finished tests for swiftpm ---
--- Installing swiftpm ---
```

was printed at the end of the CI job log with no actual build phase output in between these markers. Now these build phase markers are printed in the correct order and one can infer which log messages were printed for each build phase.

Author: MaxDesiatov

utils/swift_build_support/swift_build_support/build_script_invocation.py

@@ -891,20 +891,20 @@ def execute_product_build_steps(self, product_class, host_target):
             build_dir=build_dir)
         if product.should_clean(host_target):
             log_message = "Cleaning %s" % product_name
-            print("--- {} ---".format(log_message))
+            print("--- {} ---".format(log_message), flush=True)
             with log_time_in_scope(log_message):
                 product.clean(host_target)
         if product.should_build(host_target):
             log_message = "Building %s" % product_name
-            print("--- {} ---".format(log_message))
+            print("--- {} ---".format(log_message), flush=True)
             with log_time_in_scope(log_message):
                 product.build(host_target)
         if product.should_test(host_target):
             log_message = "Running tests for %s" % product_name
-            print("--- {} ---".format(log_message))
+            print("--- {} ---".format(log_message), flush=True)
             with log_time_in_scope(log_message):
                 product.test(host_target)
-            print("--- Finished tests for %s ---" % product_name)
+            print("--- Finished tests for %s ---" % product_name, flush=True)
         # Install the product if it should be installed specifically, or
         # if it should be built and `install_all` is set to True.
         # The exception is select before_build_script_impl products
@@ -914,6 +914,6 @@ def execute_product_build_steps(self, product_class, host_target):
            (self.install_all and product.should_build(host_target) and
            not product.is_ignore_install_all_product()):
             log_message = "Installing %s" % product_name
-            print("--- {} ---".format(log_message))
+            print("--- {} ---".format(log_message), flush=True)
             with log_time_in_scope(log_message):
                 product.install(host_target)