Fix mill and add GitHub CI for mill (#287) (#288)

(cherry picked from commit 582bb6e)

Co-authored-by: Jiuyang Liu <liu@jiuyang.me>

.github/workflows/test.yml

@@ -19,6 +19,11 @@ jobs:
         uses: olafurpg/setup-scala@v10
       - name: Cache
         uses: coursier/cache-action@v5
+      - name: Setup Mill
+        uses: jodersky/setup-mill@v0.2.3
+      - name: Mill sanity check
+        id: mill
+        run: mill __.compile
       - name: Documentation
         id: doc
         run: sbt doc

build.sc

@@ -1,6 +1,12 @@
 // SPDX-License-Identifier: Apache-2.0
 
 // Build script for mill 0.6.0
+import mill._
+import mill.scalalib._
+import mill.scalalib.publish._
+import coursier.maven.MavenRepository
+import $ivy.`com.lihaoyi::mill-contrib-buildinfo:$MILL_VERSION`
+import mill.contrib.buildinfo.BuildInfo
 
 object treadle extends mill.Cross[treadleCrossModule]("2.11.12", "2.12.11")