Fix the plugn shell dev helper on linux

[killjoy1221]

This automatically selects a file suffix on linux based on uname -m. I've also made lowercasing the output of uname to use the bashism ${X,,}.

There's room for Apple M2 if separate binaries ever need to be built for that.

[josegonzalez]

I forget, where is this file ever used?

[killjoy1221]

I don't think it's actually included in the final build. My understanding is it was a helper script during dev so we don't have to run ./build/linux/plugn, as that's basically what the script called via exec.