Make spack environment configurations writable from spack external and spack compiler find (#18165)

* spack config: default modification scope can be an environment

The previous model was that environments are the highest priority config
scope for config reading operations, but were not considered for config
writing operations. Now, the active environment is the highest priority
config scope for both reading and writing operations.

Now spack config add, spack external find and spack compiler set environment 
configuration in the environment by default if an environment is active. This is a
change in default behavior for these routines, but better matches the mental
model for an environment taking precedence over the user's default config file.

* add scope argument to 'spack external find' to choose non-default scope

* Increase testing for config modifications on environments

Co-authored-by: Gregory Becker <becker33@llnl.gov>

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