Update INSTALL_LINUX.md

[rdeprera]

I added the installation of nodes through apt, for cases where the user configured Natron ppa repository.

Thanks!

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Description

Please provide a description of what this PR is meant to fix,
and how it works (if it's not going to be very clear from the code).

[coveralls]

Coverage remained the same at 0.0% when pulling 1a88069 on rdeprera:patch-1 into 4af0454 on NatronGitHub:RB-2.4.

[devernay]

Thank you, @rdeprera !
Unfortunately, we don't support third-party binaries, because too many bug reports come from bad third-party binaries. This is especially true for plugins, which may be compiled against different versions of the dependencies than the app. openfx-io has zillions of these. We don't recommend using third party binaries for the base set of plugins (openfx-io and openfx-misc).