[Agda] --program-suffix using stack

[Andreas Nuyts]

Hi everyone,

According to the installation instructions at
https://agda.readthedocs.io/en/latest/getting-started/installation.html
we should use `make install` to install using stack and `cabal install 
Agda-2.6.1 --program-suffix=-2.6.1` to install with suffix -2.6.1. 
Naively merging these instructions to `make install 
--program-suffix=-2.6.1` is not accepted. Can I install Agda with a 
suffix using stack?

Best regards,
Andreas