EnvironmentModifications: allow disabling stack tracing (#26706)

Currently Spack keeps track of the origin in the code of any
modification to the environment variables. This is very slow 
and enabled unconditionally even in code paths where the 
origin of the modification is never queried.

The only place where we inspect the origins of environment 
modifications is before we start a build, If there's an override 
of the type `e.set(...)` after incremental changes like 
`e.append_path(..)`, which is a "suspicious" change.

This is very rare though.

If an override like this ever happens, it might mean a package is
broken. If that leads to build errors, we can just ask the user to run
`spack -d install ...` and check the warnings issued by Spack to find
the origins of the problem.

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