Revert "Use millFile instead of ./mill"

This reverts commit fe2c71c.

This change was not needed since the working directory of the process is
already `index.app.env.workingDirectory`

Author: olafurpg

scip-java/src/main/scala/com/sourcegraph/scip_java/buildtools/MillBuildTool.scala

@@ -39,7 +39,7 @@ class MillBuildTool(index: IndexCommand) extends BuildTool("mill", index) {
     val localMill = Files.isRegularFile(millFile)
     val command =
       if (localMill) {
-        millFile.toString()
+        "./mill"
       } else {
         "mill"
       }