So far, the environment used by `command` was completely leaky and the
one used by `nix` was very leaky despite it trying to be a "hermetic"
environment.
This commit moves the hermaticity to `command` and changes its
implementation to be not leak anything.
To achieve this, the following changes were also nessecary:
- the `files` and `snapshot` fixture now use the folder `test-home`
within the tmp_path directory by default, as the `HOME` environment
variable is set to there. (extraction not possible due to dependencies
of command etc also using this directory)
Fixes: #847, #848
Change-Id: I55f86ee0e1615e73fcf442ee2f28f3b89893bbb4
