Updates to environments were not multi-process safe, which prevented them from taking advantage of parallel builds as implemented in #13100. This is a minimal set of changes to enable `spack install` in an environment to be parallelized:
- [x] add an internal lock, stored in the `.spack-env` directory,
to synchronize updates to `spack.yaml` and `spack.lock`
- [x] add `Environment.write_transaction` interface for this lock
- [x] makes use of `Environment.write_transaction` in `install`,
`add`, and `remove` commands
- `uninstall` is not synchronized yet; that is left for a future PR.
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