Reduce concurrency of scalafmtCheck

Ref scalameta/scalafmt#1399

build.sbt

@@ -10,6 +10,8 @@ ThisBuild / version := {
   val v = "1.3.0-SNAPSHOT"
   nightlyVersion.getOrElse(v)
 }
+ThisBuild / scalafmtOnCompile := !(Global / insideCI).value
+ThisBuild / Test / scalafmtOnCompile := !(Global / insideCI).value
 
 // ThisBuild settings take lower precedence,
 // but can be shared across the multi projects.
@@ -43,7 +45,6 @@ def buildLevelSettings: Seq[Setting[_]] =
       homepage := Some(url("https://github.com/sbt/sbt")),
       scmInfo := Some(ScmInfo(url("https://github.com/sbt/sbt"), "git@github.com:sbt/sbt.git")),
       resolvers += Resolver.mavenLocal,
-      scalafmtOnCompile := true,
     )
   )
 

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,
+    )
+}