Make SCons spawn execute overrides chainable

Currently two scons tools (build metrics and oom_auto_retry) use a pattern to override the spawn function. This allows one full control over the CommandAction subprocess that scons normally spawns for many of its command lines.

However the current pattern does not lend a away to chain or combine these two tools together. The current pattern will overwrite any existing spawn overrides.

It would be useful if a way to chain these could be devised, most likely need some one to pass the subprocess around.