Pin conda-build to 3.16 to avoid...

Pin conda-build to 3.16 to avoid (https://github.com/conda/conda-build/issues/3289), see beat/beat.core!43

gitlab/before_build.sh

@@ -43,7 +43,7 @@ echo "Contents of \`${CONDARC}':"
 cat ${CONDARC}
 
 # updates conda installation
-run_cmd ${CONDA_ROOT}/bin/conda install python conda=4 curl conda-build=3
+run_cmd ${CONDA_ROOT}/bin/conda install python conda=4 curl conda-build=3.16
 
 # cleans up
 run_cmd ${CONDA_ROOT}/bin/conda clean --lock