Unset GREP_OPTIONS at the start of the build.

Custom grep options set in the environment variables can break the build, e.g. note the line number (-n) in the following output

     ===== Processing 'gold' =====
     ===== Cloning from 2:git@github.com:t-crest/patmos-gold.git =====
    Cloning into /home/jamie/patmos/patmos-misc/gold...
    ssh: connect to host 2 port 22: Invalid argument
    fatal: The remote end hung up unexpectedly