fix using shipped stdlib depot (#1400)

[skip ci]

Author: IanButterworth

.ci/agent_startup.conf

@@ -6,7 +6,7 @@ StartLimitBurst       = 0
 [Service]
 EnvironmentFile       = ${SRC_DIR}/.env
 Environment           = AGENT_IDX=${AGENT_IDX}
-Environment           = JULIA_DEPOT_PATH=${AGENT_DIR}/depot
+Environment           = JULIA_DEPOT_PATH=${AGENT_DIR}/depot:
 Environment           = BINARYBUILDER_STORAGE_DIR=${AGENT_DIR}/storage
 Environment           = TMPDIR=${AGENT_DIR}/tmp
 WorkingDirectory      = ${AGENT_DIR}