Update linux_install.md

[franticspider]

I've just installed tidal-cycles on a clean Ubuntu 20.04 - suggest the following changes

• put the warning about SC3 plugins before the instructions, so people don't try and install it when they don't need to (like I did)
• removed --lib from the line cabal install tidal, because I couldn't get it to work with that option

[ndr-brt]

I think the --lib option depends on the cabal version you have installed. It's something that was introduced from day to night, it was discussed here: tidalcycles/Tidal#572 (comment)
I would not remove that from the documentation, instead I would try to differentiate the two processed based on the cabal version if possible