Changes

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