Fix build issue caused by scala-xml version conflict

The build failed with
```
[error] java.lang.RuntimeException: found version conflict(s) in library dependencies; some are suspected to be binary incompatible:
[error]
[error]         * org.scala-lang.modules:scala-xml_2.12:2.1.0 (early-semver) is selected over {1.0.6}
[error]             +- org.scoverage:scalac-scoverage-reporter_2.12:2.0.7 (depends on 2.1.0)
```

See also scala/bug#12632

Author: magro

project/plugins.sbt

@@ -1,3 +1,7 @@
+ThisBuild / libraryDependencySchemes ++= Seq(
+  "org.scala-lang.modules" %% "scala-xml" % VersionScheme.Always
+)
+
 addSbtPlugin("org.scoverage" % "sbt-scoverage" % "2.0.6")
 addSbtPlugin("com.typesafe.sbt" % "sbt-site" % "1.4.1")
 addSbtPlugin("com.github.sbt" % "sbt-ghpages" % "0.7.0")