renamed "args.toolchains" to "args.toolchain" because that looks better in the
help output:

```
./mpsd-software.py install --help
usage: mpsd-software.py install [-h] [--enable-build-cache] release toolchain [toolchain ...]
```

is better than

```
./mpsd-software.py install --help
usage: mpsd-software.py install [-h] [--enable-build-cache] release toolchains [toolchains ...]
```

because I wanted to know if package sets are included in the return value

- if passed to 'available', this creates a long and not very helpful message
  that lists all branches.
- if we catch it here, we avoid that.

amongst other things explain why 'available' is called 'available'

Can easily re-add when the feature comes. The argument is not used in the
function yet at all. It is not even clear this is the right place to clear the
environment before we call 'install' again.

(Function takes one argument less now after
https://gitlab.gwdg.de/mpsd-cs/mpsd-software-environments/-/merge_requests/49)

(https://gitlab.gwdg.de/mpsd-cs/mpsd-software-environments/-/issues/9)

as they do not work; their existence will only cause confusion