jenkins.sh: Workaround libusrp build race conditon

When submitting patches to osmo-trx in gerrit, sometimes the jenkins job
fails due to libusrp failing to build. I never have this issue in on my
workstation.

Let's disable parallel make and see if we can avoid it for now.

Related: OS#3970
Change-Id: I24bc54f5872e8edb9fab5b88055a00cebe1a6911
1 file changed