build-qemu-robot-docker: pin robotframework to v6

A new major release of the robotframework was recently released (v7).
This new release appears to not be playing well with our jenkins code
update job. The robot plugin is showing the following fail:

```
Archiving artifacts
Robot results publisher started...
-Parsing output xml:
Done!
-Copying log files to build dir:
Done!
-Assigning results to build:
ERROR: Build step failed with exception
java.lang.NullPointerException
Build step 'Publish Robot Framework test results' marked build as failure
```

Until we understand this better, pin our CI to the v6 level

Change-Id: Ibe4f11a04fa8af5c435a02635916106c59419196
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
1 file changed