scons: Copy the value of "tags" before adding "add_tags" to it.

Otherwise the tags in "add_tags" are added into the original, modifying
it for the caller.

Change-Id: I6a82168d54f80ec9f919825edebe7f0ef154c5ae
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/51830
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

site_scons/gem5_scons/sources.py

@@ -201,7 +201,7 @@ class SourceFile(object, metaclass=SourceMeta):
             tags = { tags }
         if not isinstance(tags, set):
             tags = set(tags)
-        self.tags = tags
+        self.tags = tags.copy()
 
         if add_tags:
             if isinstance(add_tags, str):