vendor._gowin: use braces around net names in SDC

Gowin's SDC parser doesn't like quotes, so use braces instead.

This is a follow-up to PR #1501, merged as 0f27429. That PR just missed
this one last use of tcl_quote.

Author: agrif

amaranth/vendor/_gowin.py

@@ -448,7 +448,7 @@ def _osc_div(self):
         "{{name}}.sdc": r"""
         // {{autogenerated}}
         {% for signal, frequency in platform.iter_signal_clock_constraints() -%}
-            create_clock -name {{ "{" }}{{signal.name}}{{ "}" }} -period {{1000000000/frequency}} [get_nets {{signal|hierarchy("/")|tcl_quote}}]
+            create_clock -name {{ "{" }}{{signal.name}}{{ "}" }} -period {{1000000000/frequency}} [get_nets {{ "{" }}{{signal|hierarchy("/")}}{{ "}" }}]
         {% endfor %}
         {% for port, frequency in platform.iter_port_clock_constraints() -%}
             create_clock -name {{ "{" }}{{port.name}}{{ "}" }} -period {{1000000000/frequency}} [get_ports