Resolve PREFIX to absolute path

I think it makes sense to resolve or reject relative PREFIX paths. Currently, the script does much of its work only to then fail with obscure message and delete all the evidence of the problem. I spent quite some time figuring that out.

setup.sh

@@ -3,7 +3,7 @@
 set -euo pipefail
 
 readonly FLAVOUR="${FLAVOUR:-gmp}"
-readonly PREFIX="${PREFIX:-$HOME/.ghc-wasm}"
+readonly PREFIX="$(realpath "${PREFIX:-$HOME/.ghc-wasm}")"
 readonly REPO=$PWD
 readonly SKIP_GHC="${SKIP_GHC:-}"