[bin/utils.sh] Now set ${OS} with Operating System as the value

[missingcharacter]

I am on Linux amd64 and was getting this error:

$ asdf list flutter 3.7.0-stable
No compatible versions installed (flutter 3.7.0-stable)

with these changes I was able to install flutter

[fejzuli]

Actually now that I look at it a little closer I don't believe the proposed changes have anything in common with the initial problem, but it's still nice to fail properly when on an unsupported kernel.

@missingcharacter for the initial problem you want to do

asdf list all flutter 3.7.0-stable

if you want to check if is downloadable. Without the all it checks if it's installed locally