Append swiftc flags passed from the environment to the executable phase

This allows for SWIFTCFLAGS to be specified such that building the unit
test product can adopt specific -I paths for finding modules like XCTest

Author: phausler

lib/script.py

@@ -117,6 +117,7 @@ def generate_products(self):
             swift_flags += "-g -Onone "
         elif Configuration.current.build_mode == Configuration.Release:
             swift_flags += " "
+        swift_flags += Configuration.current.extra_swift_flags