Bad avrdude path in arduino-PR-beta1.9-BUILD-31 on Linux

This seems to be a repeat of an older bug like: https://github.com/arduino/Arduino/issues/4110

```
java.io.IOException: Cannot run program "{runtime.tools.avrdude.path}/bin/avrdude": error=2, No such file or directory
at java.lang.ProcessBuilder.start(ProcessBuilder.java:1048)
at java.lang.Runtime.exec(Runtime.java:620)
at java.lang.Runtime.exec(Runtime.java:485)
at processing.app.helpers.ProcessUtils.exec(ProcessUtils.java:11)
at cc.arduino.packages.Uploader.executeUploadCommand(Uploader.java:129)
at cc.arduino.packages.uploaders.SerialUploader.uploadUsingPreferences(SerialUploader.java:207)
at cc.arduino.UploaderUtils.upload(UploaderUtils.java:78)
at processing.app.SketchController.upload(SketchController.java:796)
at processing.app.SketchController.exportApplet(SketchController.java:767)
at processing.app.Editor$DefaultExportHandler.run(Editor.java:2241)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.io.IOException: error=2, No such file or directory
at java.lang.UNIXProcess.forkAndExec(Native Method)
at java.lang.UNIXProcess.(UNIXProcess.java:247)
at java.lang.ProcessImpl.start(ProcessImpl.java:134)
at java.lang.ProcessBuilder.start(ProcessBuilder.java:1029)
... 10 more
An error occurred while uploading the sketch
```
Laptop used: Fedora 27, 64bit Intel.

Workaround change Downloads/arduino-PR-beta1.9-BUILD-31/hardware/arduino/avr/platform.txt
from:
```
tools.avrdude.path={runtime.tools.avrdude.path}
```
to:
```
tools.avrdude.path=/home/YourUser/Downloads/arduino-PR-beta1.9-BUILD-31/hardware/tools/avr/
```
and uploads now work.

Start with hardware/arduino/avr/platform.txt in the arduino-PR-beta1.9-BUILD-31 package and inspect the tools.avrdude.path setting shown in the report. Trace the upload path from the Uploader and SerialUploader stack frames, then verify on Linux that uploading no longer reports a missing avrdude executable.

Written by the indexing model from the issue text.