osmo-gsm-tester: Build srsLTE inside the docker instance if inst not provided by host osmo-gsm-tester.git/contrib/jenkins-build-srslte.sh cannot be used because there's a need to split the fetch step from the build step. That's because we want to fetch in the host (in order to use host's user ssh key for potential private repos) while still building inside the docker container, where we have all the dependencies (and where the binaries are going to be run afterwards by osmo-gsm-tester). During the changes above it was also noticed that /tmp/trial on the host got its UID+GID changed (because we switch user inside the container to downscale privileges). Fix it by chown'ing the directory back to the host user's UID+GID. Change-Id: Ia0db8fba840392a0b7883cfc15e45c2e80407a16
