Build: allow multi-line commands on build.commands

[humitos]

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:

• Create user facing documentation structure pypi/warehouse#12953
• Create user facing documentation structure pypi/warehouse#12953

Closes #10103
Closes #10208

[ericholscher]

This seems... too simple. Do we know why this works? Agreed it seems worth trying if it solves the issue, but I feel like we probably want some kind of more robust solution, but baby steps :)

[bensze01]

@ericholscher It works because the PATH variable is always implicitly exported by the shell.

so PATH=foo command, PATH=foo; command and PATH=foo\ncommand produce identical results.

[bensze01]

To quote the POSIX spec:

2.5.3 Shell Variables

Variables shall be initialized from the environment (as defined by XBD Environment Variables and the exec function in the System Interfaces volume of POSIX.1-2017) and can be given new values with variable assignment commands. If a variable is initialized from the environment, it shall be marked for export immediately; see the export special built-in.

[ericholscher]

@bensze01 Thanks for the explanation 💯