Fix escaping in interpolated string for tool runner

```
scala> val tool = "scalap"
tool: String = scalap

scala> s"cmd.exe /c tools\$tool.bat -p"
scala.StringContext$InvalidEscapeException: invalid escape at terminal index 16 in "cmd.exe /c tools\". Use \\ for literal \.
  at scala.StringContext$.loop$1(StringContext.scala:211)
  at scala.StringContext$.replace$1(StringContext.scala:246)
  at scala.StringContext$.treatEscapes0(StringContext.scala:250)
  at scala.StringContext$.treatEscapes(StringContext.scala:195)
  ...

scala> s"cmd.exe /c tools\\$tool.bat -p"
res2: String = cmd.exe /c tools\scalap.bat -p
```

Author: retronym

build.sbt

@@ -254,7 +254,7 @@ lazy val generateVersionPropertiesFileImpl: Def.Initialize[Task[File]] = Def.tas
   def executeTool(tool: String) = {
       val cmd =
         if (System.getProperty("os.name").toLowerCase.contains("windows"))
-          s"cmd.exe /c tools\$tool.bat -p"
+          s"cmd.exe /c tools\\$tool.bat -p"
         else s"tools/$tool"
       Process(cmd).lines.head
   }