Use real paths for zinc base path

The AppConfiguration.baseDirectory is dealiased during project loading.
Not dealiasing the symlink here could cause a discrepancy between the
`baseDirectory` key and the value of the base key in the root paths map.

main/src/main/scala/sbt/Defaults.scala

@@ -191,7 +191,7 @@ object Defaults extends BuildCommon {
       allowMachinePath :== true,
       rootPaths := {
         val app = appConfiguration.value
-        val base = app.baseDirectory
+        val base = app.baseDirectory.getCanonicalFile
         val boot = app.provider.scalaProvider.launcher.bootDirectory
         val ih = app.provider.scalaProvider.launcher.ivyHome
         val coursierCache = csrCacheDirectory.value