stdexec: update to later commit

The upstream code has done some namespace changes.  Point to a commit
in that timeframe and fix up the local usages in order to compile
successfully.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I5a21ccd0774ee5cd22d45c9a425e43560734a571
diff --git a/src/async/context.cpp b/src/async/context.cpp
index bbb198e..2f6454d 100644
--- a/src/async/context.cpp
+++ b/src/async/context.cpp
@@ -142,7 +142,7 @@
     wait_for_wait_process_stopped();
 
     // Wait for all the internal tasks to complete.
-    std::this_thread::sync_wait(
+    stdexec::this_thread::sync_wait(
         internal_tasks.empty() | execution::upon_error([&](auto&& e) {
             pending_exceptions.emplace_back(std::move(e));
         }));