Perhaps this was added later, but this would quickly remove all packages in the environment.
using Pkg
pkg"rm --all"
[rm|remove] [-p|--project] pkg[=uuid] ...
[rm|remove] [-p|--project] [--all]
Remove package pkg from the project file. Since the name pkg can only refer
to one package in a project this is unambiguous, but you can specify a uuid
anyway, and the command is ignored, with a warning, if package name and UUID
do not match. When a package is removed from the project file, it may still
remain in the manifest if it is required by some other package in the
project. Project mode operation is the default, so passing -p or --project
is optional unless it is preceded by the -m or --manifest options at some
earlier point. All packages can be removed by passing --all.
[rm|remove] [-m|--manifest] pkg[=uuid] ...
[rm|remove] [-m|--manifest] [--all]
Remove package pkg from the manifest file. If the name pkg refers to
multiple packages in the manifest, uuid disambiguates it. Removing a package
from the manifest forces the removal of all packages that depend on it, as
well as any no-longer-necessary manifest packages due to project package
removals. All packages can be removed by passing --all.
Also perhaps you like the current environment and would simply not have it be the default environment. You might be interested in