Add caching for the Mill build tool

__tests__/cache/mill/.gitignore

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+out/

__tests__/cache/mill/.mill-version

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+0.12.3

__tests__/cache/mill/build.sc

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+package build
+import mill._, scalalib._
+
+object MyProject extends ScalaModule {
+    def scalaVersion = "2.13.11"
+    def ivyDeps = Agg(ivy"com.lihaoyi::mainargs:0.6.2")
+
+    object test extends ScalaTests {
+        def ivyDeps = Agg(ivy"com.lihaoyi::utest:0.8.5")
+        def testFramework = "utest.runner.Framework"
+    }
+}

__tests__/cache/mill/mill

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+#!/usr/bin/env sh
+# This is a wrapper script that automatically downloads Mill from GitHub.
+set -e
+
+if [ -z "$MILL_VERSION" ] ; then
+    MILL_VERSION="$(head -n 1 .mill-version 2> /dev/null)"
+fi
+
+MILL_DOWNLOAD_PATH="$HOME/.cache/mill/download"
+MILL_EXEC_PATH="${MILL_DOWNLOAD_PATH}/$MILL_VERSION"
+
+if [ ! -x "$MILL_EXEC_PATH" ] ; then
+    mkdir -p "${MILL_DOWNLOAD_PATH}"
+    DOWNLOAD_FILE=$MILL_EXEC_PATH-tmp-download
+    MILL_DOWNLOAD_URL="https://github.com/lihaoyi/mill/releases/download/${MILL_VERSION%%-*}/$MILL_VERSION-assembly"
+    curl --fail -L -o "$DOWNLOAD_FILE" "$MILL_DOWNLOAD_URL"
+    chmod +x "$DOWNLOAD_FILE"
+    mv "$DOWNLOAD_FILE" "$MILL_EXEC_PATH"
+    unset DOWNLOAD_FILE
+    unset MILL_DOWNLOAD_URL
+fi
+
+unset MILL_DOWNLOAD_PATH
+unset MILL_VERSION
+
+exec "${MILL_EXEC_PATH}" "$@"