Fix bugs in example of how to use with pipenv (#607)

* Fix bugs in example of how to use with pipenv

The current example of how to use `@actions/cache` with pipenv has two
problems:

 1. The cached virtualenv that pipenv creates has `bin/python` as a symlink
    into paths like `/opt/hostedtoolcache/Python/3.7.11` that explicitly
    include the patch version of python. When the cache is restored onto a
    machine running a slightly different version of python, e.g., when
    GitHub upgrades its runners from python 3.7.10 to 3.7.11, then any
    attempt to run python in the workflow mysteriously fails with errors
    like `Failed to load paths: /bin/sh: 1: /home/runner/.local/share/virtualenvs/myrepo-sOIMCiTO/bin/python: not found`.

    Therefore the patch version of python should be included in the cache
    key.

 2. `pipenv --install` has the unfortunate behaviour of not cleaning out
    any pre-existing packages. That is, if the `Pipfile` first contains
    dependencies on `foo` and `bar`, and then you remove `bar` from the
    `Pipfile` and run `pipenv install` again, `bar` is still included in
    the virtualenv.

    This can cause false-positive test failures: when a dependency is
    removed from the `Pipfile` but there is still code that relies on the
    removed dependency, tests can still pass if the dependency comes from
    the cache based on a previous revision of `Pipfile.lock`.

    Therefore `restore-keys` should not be set.

This PR attempts to address both of these issues.

* Explain why setup-python is included in example

examples.md

@@ -435,12 +435,17 @@ jobs:
 ## Python - pipenv
 
 ```yaml
+- name: Set up Python
+ # The actions/cache step below uses this id to get the exact python version
+ id: setup-python
+ uses: actions/setup-python@v2
+
+ ⋮
+
 - uses: actions/cache@v2
  with:
  path: ~/.local/share/virtualenvs
- key: ${{ runner.os }}-pipenv-${{ hashFiles('Pipfile.lock') }}
- restore-keys: |
- ${{ runner.os }}-pipenv-
+ key: ${{ runner.os }}-python-${{ steps.setup-python.outputs.python-version }}-pipenv-${{ hashFiles('Pipfile.lock') }}
 ```
 
 ## R - renv