Update tox example 2 to set install_command

As suggested by @finswimmer, use `install_command = ...` to pass
`--no-deps` to pip in the second tox usage example. This skips an
unnecessary step in which pip installs dependencies which poetry will
then handle in the sync step.

Additionally, normalize the style of commands a bit in the proposed
update to Usecase 2. Commands are no longer inlined with the config
key, which makes the example slightly longer, but also more consistent
with the other examples.

Author: sirosen

docs/faq.md

@@ -82,24 +82,31 @@ isolated_build = true
 
 [testenv]
 allowlist_externals = poetry
-commands_pre = poetry install --no-root --sync --with test
-commands = pytest tests/
+install_command =
+    python -m pip install --no-deps {opts} {packages}
+commands_pre =
+    poetry install --no-root --sync --with test
+commands =
+    pytest tests/
 
 [testenv:style]
 skip_install = true
-commands_pre = poetry install --no-root --sync --with style
+commands_pre =
+    poetry install --no-root --sync --with style
 commands =
     flake8 src/
     black --check src/
 
 [testenv:typing]
-commands_pre = poetry install --no-root --sync --with typing
-commands = mypy src/
+commands_pre =
+    poetry install --no-root --sync --with typing
+commands =
+    mypy src/
 ```
 
-`tox` will create an `sdist` package of the project and uses `pip` to install it in a fresh environment.
-Thus, dependencies are resolved by `pip` in the first place. But afterwards we run Poetry,
-which will install the locked dependencies into the environment.
+`tox` will create an `sdist` package of the project and uses `pip` to install it in a fresh environment,
+but because `--no-deps` is used, `pip` will not resolve any dependencies.
+Afterwards we run Poetry, which will install the locked dependencies into the environment.
 
 In the `style` testenv, `tox` will only create the virtual environment, and
 again we will rely on Poetry to install requirements. The project itself will