Build: allow multi-line commands on build.commands

This is another attempt to fix this issue. It follows the suggestion posted by
@bensze01 in #10103. It
looks simple and I think it makes sense to give it a try since it doesn't seem
to be harmful.

I tested it locally with the branch
https://github.com/readthedocs/test-builds/tree/build-commands-multiline and it
worked fine. Note that without this PR the branch linked fails.

Related #10133
Related #10119
Related #10206

External PRs that should be solved by this one:
* pypi/warehouse#12953
* pypi/warehouse#12953

Closes #10103
Closes #10208

Author: humitos

readthedocs/doc_builder/environments.py

@@ -376,7 +376,7 @@ def get_wrapped_command(self):
                 for part in self.command
             )
         )
-        return "/bin/sh -c '{prefix}{cmd}'".format(
+        return "/bin/sh -c '{prefix}\n{cmd}'".format(
             prefix=prefix,
             cmd=command,
         )