If the nix store lives on NFS, `ghc 8.2.1` is unable to build a package
database. This bug was fixed by @bgamari in `ghc 8.2.2` here:
https://ghc.haskell.org/trac/ghc/ticket/13945
This commit upgrades the unpacked bootstrap GHC version, so that we can build
newer versions of GHC even if the store is on NFS.
Benjamin Hipple