add 'tidy' to Debian images

[jameslamb]

Since #62 was merged, I've observed that my package ({lightgbm}) has one NOTE from R CMD check when running on the Debian images.

* checking HTML version of manual ... NOTE
Skipping checking HTML validation: no command 'tidy' found
...
Status: 1 NOTE
...
NOTEs, WARNINGs, or ERRORs have been found by R CMD check

ref: microsoft/LightGBM#5587

A bit hard to tell, but I think this is related to some changes on r-devel's use of tidy (link) that happened around late July (this commit).

This PR proposes adding tidy to the Debian images to resolve that NOTE.

Notes for Reviewers

I expect that CRAN has tidy installed on its Linux machines, based on the following line in Section 1.3.1 of "Writing R Extensions" (link).

Optionally (including by R CMD check --as-cran) the HTML version of the manual is created and checked for compliance with the HTML5 standard. This requires a recent version52 of ‘HTML Tidy’, either on the path or at a location specified by environment variable R_TIDYCMD. Up-to-date versions can be installed from http://binaries.html-tidy.org/.

Thanks for your time and consideration.

[gaborcsardi]

Thanks!