Build fails if `cache_file` points to an unexisting folder

[Guts]

First of all, thank you for this very useful and comprehensive plugin! Nice work!

I'm facing an issue when trying to store the cache_file into a .cache/plugins/encryptcontent/ folder that does not exist at build time.

To reproduce

1. Set cache_file to a path that does not exist yet:

   plugins:
   [...]
   - encryptcontent:
       global_password: !ENV MKDOCS_PLUGIN_ENCRYPT_PASSWORD
       cache_file:
         !ENV [
           MKDOCS_PLUGIN_ENCRYPT_CACHE_FILE,
           .cache/plugins/encryptcontent/encryptcontent.cache,
         ]
       title_prefix: ""
       password_button: true
       password_button_text: "Unlock content"
       remember_keys: true
       remember_password: true
       remember_prefix: encryptcontent_plugin_igv_scpo

2. build: mkdocs build

It fails.

Error log

Traceback (most recent call last):
  File "/home/chapichapo/.venv/bin/mkdocs", line 8, in <module>
    sys.exit(cli())
  File "/home/chapichapo/.venv/lib/python3.10/site-packages/click/core.py", line 1157, in __call__
    return self.main(*args, **kwargs)
  File "/home/chapichapo/.venv/lib/python3.10/site-packages/click/core.py", line 1078, in main
    rv = self.invoke(ctx)
  File "/home/chapichapo/.venv/lib/python3.10/site-packages/click/core.py", line 1688, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/chapichapo/.venv/lib/python3.10/site-packages/click/core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/chapichapo/.venv/lib/python3.10/site-packages/click/core.py", line 783, in invoke
    return __callback(*args, **kwargs)
  File "/home/chapichapo/.venv/lib/python3.10/site-packages/mkdocs/__main__.py", line 286, in build_command
    build.build(cfg, dirty=not clean)
  File "/home/chapichapo/.venv/lib/python3.10/site-packages/mkdocs/commands/build.py", line 354, in build
    config.plugins.on_post_build(config=config)
  File "/home/chapichapo/.venv/lib/python3.10/site-packages/mkdocs/plugins.py", line 542, in on_post_build
    return self.run_event('post_build', config=config)
  File "/home/chapichapo/.venv/lib/python3.10/site-packages/mkdocs/plugins.py", line 509, in run_event
    result = method(**kwargs)
  File "/home/chapichapo/.venv/lib/python3.10/site-packages/encryptcontent/plugin.py", line 1117, in on_post_build
    with open(self.setup['cache_file'], 'w') as stream:
FileNotFoundError: [Errno 2] No such file or directory: '/home/chapichapo/.cache/plugins/encryptcontent/encryptcontent.cache'

Workaround

Create the folder before.

[unverbuggt]

Hi, thank you for pointing this out. I'll add a few lines to create the folder if it doesn't exist in the next release.