Reduce concurrency of scalafmtCheck

Ref scalameta/scalafmt#1399

build.sbt

@@ -27,8 +27,8 @@ ThisBuild / version := {
   }
 }
 ThisBuild / licenses := List(("Apache-2.0", url("https://www.apache.org/licenses/LICENSE-2.0")))
-ThisBuild / scalafmtOnCompile := true
-ThisBuild / Test / scalafmtOnCompile := true
+ThisBuild / scalafmtOnCompile := !(Global / insideCI).value
+ThisBuild / Test / scalafmtOnCompile := !(Global / insideCI).value
 
 def buildLevelSettings: Seq[Setting[_]] = Seq(
   // https://github.com/sbt/sbt-git/issues/109

project/FixScalafmtPlugin.scala

@@ -0,0 +1,23 @@
+import sbt._
+import Keys._
+import org.scalafmt.sbt.ScalafmtPlugin
+import ScalafmtPlugin.autoImport._
+
+object FixScalafmtPlugin extends AutoPlugin {
+  override def requires = ScalafmtPlugin
+  override def trigger = allRequirements
+
+  val ScalaFmtTag = ConcurrentRestrictions.Tag("scalafmt")
+
+  override def globalSettings: Seq[Def.Setting[_]] =
+    Seq(
+      concurrentRestrictions += Tags.limit(ScalaFmtTag, 1)
+    )
+
+  override def projectSettings: Seq[Def.Setting[_]] =
+    Seq(
+      scalafmtCheckAll := (scalafmtCheckAll.tag(ScalaFmtTag)).value,
+      Compile / scalafmtCheck := ((Compile / scalafmtCheck).tag(ScalaFmtTag)).value,
+      Test / scalafmtCheck := ((Test / scalafmtCheck).tag(ScalaFmtTag)).value,
+    )
+}