[NTG-context] Installing on arm64

[Sytse Knypstra]

I followed Lutz Haseloff's suggestion and inserted

PLATFORM="linux-aarch64"

after the last esac in install.sh and indeed it installed ConTeXt LMTX.

I assume the insertion of the next two lines in the install.sh file on 
line 50 would solve the problem for future arm64 users (I didn't check):

             aarch64)
                 PLATFORM="linux-aarch64" ;;

Hans and Lutz, thank you for your advice.

Sytse

P.S. This afternoon I accidentally sent this e-mail to a wrong address; 
I now see that Lutz proposes the same patch.