Update file set_oidc_port.sh.j2: set as envvar

Setting this as envvar will make it easier to find and save us the dirty trick of grepping after cutting the listen-address string.

RAIL_PORT=$(ssh login-01.bgo1.test.rail.uhdc.no grep -- --listen-address=127.0.0.1: .kube/config | cut -d: -f2)

vs

RAIL_PORT=$(ssh login-01.bgo1.test.rail.uhdc.no echo \$OIDC_PORT)