vendor._gowin: fix clock name quotes for Gowin IDE

Gowin IDE seems unhappy with quotes in clock signal names. For instance,
with the following SDC:

create_clock -name "clk50_0__io" -period 20.0 [get_ports
    {clk50_0__io}]

Gowin IDE will complain with the following error message:

ERROR  (TA2000) : "top.sdc":2 | 'syntax error' near token '-period'

Changing quotes with curly braces fixes the issue.

amaranth/vendor/_gowin.py

@@ -437,13 +437,15 @@ def _osc_div(self):
             file delete -force {{name}}.fs
             file copy -force impl/pnr/project.fs {{name}}.fs
         """,
+        # Gowin is using neither Tcl nor the Synopsys code to parse SDC files, so the grammar
+        # deviates from the usual (eg. no quotes, no nested braces).
         "{{name}}.sdc": r"""
         // {{autogenerated}}
         {% for signal, frequency in platform.iter_signal_clock_constraints() -%}
-            create_clock -name {{signal.name|tcl_quote}} -period {{1000000000/frequency}} [get_nets {{signal|hierarchy("/")|tcl_quote}}]
+            create_clock -name {{ "{" }}{{signal.name}}{{ "}" }} -period {{1000000000/frequency}} [get_nets {{signal|hierarchy("/")|tcl_quote}}]
         {% endfor %}
         {% for port, frequency in platform.iter_port_clock_constraints() -%}
-            create_clock -name {{port.name|tcl_quote}} -period {{1000000000/frequency}} [get_ports
+            create_clock -name {{ "{" }}{{port.name}}{{ "}" }} -period {{1000000000/frequency}} [get_ports
             {{ "{" }}{{port.name}}{{ "}" }}]
         {% endfor %}
         {{get_override("add_constraints")|default("// (add_constraints placeholder)")}}