Instead of prepending all the commands with the `PATH=` variable, we pass the
environment variable directly to the Docker container.

This allow us to run multi-line shell script without failing with weird syntax
errors. Besides, the implementation is cleaner since all the environment
variables are passed to the commands in the same way.

I added some _default paths_ that I found by checking the local Docker
container. I'm also passing the users' path, depending if we are working locally
as root or in production.

This is not 100% complete and there may be some other issues that I'm not seeing
yet, but I think it's a first step to behave in a way our users are expecting.

Closes #10103

Locally it tries to reverted back 🤷

…s/readthedocs.org into humitos/hosting-integrations

X-RTD-Hosting-Integration: true/false

This can be used from CloudFlare to decide whether or not inject a `<script>`
into the resulting HTML or not.

….json`

See https://github.com/humitos/readthedocs-client

I added a `Version.build_data` field that may be used from
`/_/readthedocs-config.json` to extend with data generated by the doctool at
build time if necessary.

…mitos/hosting-integrations

This file will be deployed directly into S3.

…mitos/hosting-integrations