fix: Avoid frequent writes to graph.json in a loop

Before this change, we would write the `graph.json` over and over again for every single dependency that's added while the dependency graph is being created.
This commit changes this behavior to only write to the file once, only when all dependencies have been already added.

Signed-off-by: Michael Yochpaz <[email protected]>

Author: MichaelYochpaz

src/fromager/bootstrapper.py

@@ -951,7 +951,6 @@ def _add_to_graph(
             download_url=download_url,
             pre_built=pbi.pre_built,
         )
-        self.ctx.write_to_graph_to_file()
 
     def _sort_requirements(
         self,

src/fromager/commands/bootstrap.py

@@ -185,6 +185,8 @@ def bootstrap(
             progressbar.update()
             requirement_ctxvar.reset(token)
 
+    wkctx.write_to_graph_to_file()
+
     constraints_filename = wkctx.work_dir / "constraints.txt"
     if skip_constraints:
         logger.info("skipping constraints.txt generation as requested")