[CI] Consider sending the nightly build report in HTML format

[asfimport]

We're having an increasing number of nightly builds which makes the nightly report harder to read. Not mentioning the long URLs we have which is hard to express in a plaintext email.

The Apache mailing lists prefer plaintext format 1, though we could make an exception for the nightly build report (assuming it wouldn't bounce).

Reporter: Krisztian Szucs / @kszucs

_{Note: This issue was originally created as ARROW-12822. Please see the migration documentation for further details.}

[asfimport]

Mauricio 'Pachá' Vargas Sepúlveda / @pachadotdev:
i'm trying to send an email but it keeps bouncing, I shall try with PUC's account

[asfimport]

Antoine Pitrou / @pitrou:
cc @raulcd