octave: make qt@5.7 optional on Linux
octave: make qt@5.7 optional on Linux

The qt@5.7 formula doesn't currently build on Linux.

pditommaso/homebrew-science,heuermh/homebrew-science,philippbayer/homebrew-science,quantumsteve/homebrew-science,andrewjpage/homebrew-science,maplesond/homebrew-science,dpo/homebrew-science