Fix variable interpolation for obs-installability parameter

Signed-off-by: Luis Araujo <luis.araujo@collabora.co.uk>

obs-installability/Jenkinsfile

@@ -18,7 +18,7 @@ pipeline {
           test -n "${params.RELEASE}" && RELEASE=${params.RELEASE}
 
           export HOME=/tmp
-          ./obs-installability/obs-installability $RELEASE junit.xml
+          ./obs-installability/obs-installability \$RELEASE junit.xml
         """
       }
     }