Environment variable to skip client build.

Need to fix https://github.com/galaxyproject/planemo/issues/750 but I don't want to send along command-line arguments that will break older Galaxies, better just to set an environment variable that will be ignored for older Galaxies I think.

scripts/common_startup.sh

@@ -18,7 +18,7 @@ FETCH_WHEELS=1
 CREATE_VENV=1
 REPLACE_PIP=$SET_VENV
 COPY_SAMPLE_FILES=1
-SKIP_CLIENT_BUILD=0
+SKIP_CLIENT_BUILD=${GALAXY_SKIP_CLIENT_BUILD:-0}
 
 for arg in "$@"; do
     [ "$arg" = "--skip-eggs" ] && FETCH_WHEELS=0