environments: only write when necessary (#13546)

This changes Spack environments so that the YAML file associated with the environment is *only* written when necessary (i.e., if it is changed *by spack*).  The lockfile is still written out as before.

There is a larger question here of which part of Spack should be responsible for setting defaults in config files, and how we can get rid of empty lists and data structures currently cluttering files like `compilers.yaml`.  But that probably requires a rework of the default-setting validator in `spack.config`, as well as the code that uses `spack.config`.  This will at least help for `spack.yaml`.