Revert FairRoot version to v18.6.9-nice since mpdroot is incompatible with new...

Revert FairRoot version to v18.6.9-nice since mpdroot is incompatible with new build system of FairRoot introduced in v18.8.1.

fairroot.sh

 package: FairRoot
-version: "v18.8.1-nica"
-tag: "v18.8.1-nica"
+version: "v18.6.9-nica"
+tag: "v18.6.9-nica"
 source: https://github.com/NICAdist/FairRoot.git
 requires:
   - boost