ghc.nix no longer compatible with latest master branch

Summary

Commit 67738db1 causes builds done with ghc.nix supplying ghc-8.8.2 to fail. @tmobile

Steps to reproduce

Clone ghc.nix and run from the ghc root directory:

$ nix-shell --pure ghc.nix
$ hadrian/build -j8 --flavour=quick

Expected behavior

It should successfully build. Instead, I observe

[...]
[98 of 98] Compiling Main             ( src/Main.hs, /home/brian/mygit/ghc/hadrian/dist-newstyle/build/x86_64-linux/ghc-8.8.2/hadrian-0.1.0.0/x/hadrian/build/hadrian/hadrian-tmp/Main.o ) [Development.Shake changed]
Linking /home/brian/mygit/ghc/hadrian/dist-newstyle/build/x86_64-linux/ghc-8.8.2/hadrian-0.1.0.0/x/hadrian/build/hadrian/hadrian ...
| Configure package 'rts'
| Run Ghc CompileHs Stage0: libraries/text/Data/Text/Internal/Fusion.hs => _build/stage0/libraries/text/build/Data/Text/Internal/Fusion.o
| Configure package 'ghc'
Error when running Shake build system:
  at action, called at src/Rules.hs:40:19 in main:Rules
  at need, called at src/Rules.hs:62:5 in main:Rules
* Depends on: _build/stage0/lib/package.conf.d/ghc-8.11.0.20200529.conf
  at apply1, called at src/Development/Shake/Internal/Rules/Oracle.hs:159:32 in shake-0.18.5-d08cc570254c2937656e16b0fb9ddd0ad652646c7554b15bb3ec273551ad685b:Development.Shake.Internal.Rules.Oracle
* Depends on: OracleQ (ContextDataKey (Context {stage = Stage0, package = Package {pkgType = Library, pkgName = "ghc", pkgPath = "compiler"}, way = v}))
  at need, called at src/Hadrian/Oracles/Cabal/Rules.hs:53:9 in main:Hadrian.Oracles.Cabal.Rules
* Depends on: _build/stage0/compiler/setup-config
  at error, called at src/Hadrian/Oracles/TextFile.hs:38:42 in main:Hadrian.Oracles.TextFile
* Raised the exception:
Key 'bootstrap-threaded-rts' not found in file 'hadrian/cfg/system.config'

Environment

• GHC version used: 8.8.2 (default on ghc.nix)

Optional:

• Operating System: NixOS
• System Architecture: