fix rare faulty TEnv initialization when workdir=homedir in two Egamma packages

This implements in Egamma packages, after some delay, a suggested fix from @gstark, who found out that absolute paths (supplied by the PathResolver) are not processed by the TEnv constructor when the executable (athena / analysis framework / ...) runs directly from the home directory. Thank you!

As proposed, the updated code will instead rely upon TEnv::ReadFile(), which should work in all cases. The return code is not checked because neither was it in the TEnv constructor, so this should mimic current behaviour.

I'm adding the relevant labels to tentatively update 21.3 and 21.9 as well. For master, let's discuss separately, @turra having some interest in replacing TEnv altogether.

@christos, @okiverny