python: install python3.8-dev on Ubuntu 18.04
python: install python3.8-dev on Ubuntu 18.04

Summary:
It turns out we don't have to compile Python from source on Ubuntu
18.04. python3.8-dev is sufficient to complete a getdeps build of
Watchman on Ubuntu 18.04.

This reduces build times by a minute or two on my Vagrant VM.

Reviewed By: xavierd

Differential Revision: D38931978

fbshipit-source-id: bcde84fc5aca6e0466191650a316130c426d0dc5

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