[Release] Generate the API docs in ubuntu 20.10

[asfimport]

I noticed that the ubuntu build variable is passed as a runtime environment variable to the container: https://github.com/apache/arrow/blob/master/dev/release/post-09-docs.sh#L49

Reporter: Krisztian Szucs / @kszucs
Assignee: Krisztian Szucs / @kszucs

PRs and other links:

• GitHub Pull Request #10532

_{Note: This issue was originally created as ARROW-13080. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 10532
#10532