Introduce `no_chown` option to pip-related functions - #4805

Affected functions:
- `pip.install`
- `pip.installed`
- `virtualenv.managed`

When using `runas` in conjunction with a local-filesystem `requirements` file with these functions,
`pip.install` will create a temporary copy of the requirements file,
then change ownership of that file to the `runas` user.

The purpose of this is to avoid file permission errors when accessing the original requirements file.
However, this breaks when that file has a "-r otherfile.txt" line.

In these cases, set `no_chown` to `True`.
The original path to the requirements file will be kept,
and other requirements files included from the top-level file will be correctly found.
Merge pull request #4832 from 11craft/4805-pip-no-chown

Introduce `no_chown` option to pip-related functions