stdexec: skip build phase (#34425)

Since it's a header-only library there's nothing to build. However, the
default targets include tests and examples and there's no option to turn
them off during configuration time.

var/spack/repos/builtin/packages/stdexec/package.py

@@ -19,3 +19,6 @@ class Stdexec(CMakePackage):
 
     conflicts("%gcc@:10")
     conflicts("%clang@:13")
+
+    def build(self, spec, prefix):
+        pass