Enforce a specific conda environment for slc6 builds
LHCbNightlyConf!1156 (merged) updated the version of ccache to a binary that does not work on SLC6 giving the errors like:
.../bin/ccache: /lib64/libc.so.6: version `GLIBC_2.14' not found (required by .../bin/ccache)
This MR forces a specific version of the tools to use (the latest one know to work).