Put the NASM pulled from svn.python.org at the front of the PATH.

This will prevent failures from the version of NASM installed on buildbots,
if the ability to override the version used is desired again later, we can
figure out a way to opt in.

PCbuild/build_ssl.py

@@ -187,7 +187,8 @@ def main():
         if dir.startswith('nasm'):
             nasm_dir = os.path.join(ssl_dir, os.pardir, dir)
             nasm_dir = os.path.abspath(nasm_dir)
-            os.environ['PATH'] += os.pathsep.join(['', nasm_dir])
+            old_path = os.environ['PATH']
+            os.environ['PATH'] = os.pathsep.join([nasm_dir, old_path])
             break
     else:
         print('NASM was not found, make sure it is on PATH')