Fix Erlang/OTP version usage in CI

This commit reproduces the changes in [appsignal-elixir#718][pr]
for the Plug repository, making it so that the intended Erlang
and Elixir versions are used, by selecting the Erlang version
before the Elixir one, compiling the desired Elixir version
against the selected Erlang version, caching the compiled
Elixir version, and removing OTP 20 checks from CI.

[pr]: appsignal/appsignal-elixir#718

Author: unflxw

.semaphore/semaphore.yml

@@ -85,10 +85,6 @@ blocks:
           commands:
             - ERLANG_VERSION=21.3 ELIXIR_VERSION=1.9.4 . bin/setup
             - mix test
-        - name: Elixir 1.9.4, OTP 20
-          commands:
-            - ERLANG_VERSION=20.3 ELIXIR_VERSION=1.9.4 . bin/setup
-            - mix test
       env_vars:
         - name: MIX_ENV
           value: test

bin/setup

@@ -1,12 +1,34 @@
-if [[ $ELIXIR_VERSION == "master" ]]; then
-  kiex install $ELIXIR_VERSION
-fi
+#!/bin/bash
+
+set -e
+
+elixirs_key="elixir-$ELIXIR_VERSION-erlang-$ERLANG_VERSION-elixirs"
+archives_key="elixir-$ELIXIR_VERSION-erlang-$ERLANG_VERSION-archives"
+
+elixirs_path=~/".kiex/elixirs"
+archives_path=~/".kiex/mix/archives"
+
+rm -rf "$elixirs_path"/*
+rm -rf "$archives_path"/*
 
 sem-version erlang $ERLANG_VERSION
 erl -eval '{ok, Version} = file:read_file(filename:join([code:root_dir(), "releases", erlang:system_info(otp_release), "OTP_VERSION"])), io:fwrite(Version), halt().' -noshell
-sem-version elixir $ELIXIR_VERSION
+
+if [ $ELIXIR_VERSION != "main" ] && \
+   cache has_key "$elixirs_key" && \
+   cache has_key "$archives_key"
+then
+  cache restore "$elixirs_key"
+  cache restore "$archives_key"
+else
+  kiex install $ELIXIR_VERSION
+  cache store "$elixirs_key" "$elixirs_path"
+  cache store "$archives_key" "$archives_path"
+fi
+
+kiex use $ELIXIR_VERSION
 elixir -v
-checkout
+
 mix local.rebar --force
 mix local.hex --force
 mix deps.get