Consolidate and isolate bindist installation

In particular, this now ensure that we actually use the ghcup version we
installed as well as a separate installation directory, instead of any
potentially previously installed versions/files.

.github/workflows/install-bindist.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+set -eo pipefail
+
+export GHCUP_INSTALL_BASE_PREFIX=$RUNNER_TEMP/foobarbaz
+
+curl --proto '=https' --tlsv1.2 -sSf https://get-ghcup.haskell.org | sh
+
+source $GHCUP_INSTALL_BASE_PREFIX/.ghcup/env || source ~/.bashrc
+
+ghcup --version
+which ghcup | grep foobarbaz
+
+ghcup -v --url-source=file:$METADATA_FILE install ghc --set $GHC_VERSION
+
+ghc --version
+echo 'main = print $ 1 + 1' > main.hs
+ghc main.hs
+[[ $(./main) -eq 2 ]]