Update nixpkgs to latest master

Author: nh2

.buildkite/pipeline.yml

@@ -42,15 +42,15 @@ steps:
 
   # Different GHC versions
 
-  - label: 8.6.5 -A working
+  - label: 8.8.1 -A working
     command: |
       NIX_PATH=nixpkgs=nixpkgs nix-build --no-link survey/default.nix \
-        --argstr compiler ghc865 -A working
+        --argstr compiler ghc881 -A working
 
-  - label: 8.6.5 -A workingStackageExecutables
+  - label: 8.8.1 -A workingStackageExecutables
     command: |
       NIX_PATH=nixpkgs=nixpkgs nix-build --no-link survey/default.nix \
-        --argstr compiler ghc865 -A workingStackageExecutables
+        --argstr compiler ghc881 -A workingStackageExecutables
 
   # Stack via stack2nix
 

nixpkgs

@@ -32,15 +32,16 @@ in
   ])."${approach}",
 
   # When changing this, also change the default version of Cabal declared below
-  compiler ? "ghc864",
+  compiler ? "ghc865",
 
   defaultCabalPackageVersionComingWithGhc ?
     ({
       ghc822 = "Cabal_2_2_0_1"; # TODO this is technically incorrect for ghc 8.2.2, should be 2.0.1.0, but nixpkgs doesn't have that
       ghc844 = "Cabal_2_2_0_1";
       ghc863 = throw "static-haskell-nix: ghc863 is no longer supported, please upgrade";
-      ghc864 = "Cabal_2_4_1_0"; # TODO this is technically incorrect for ghc 8.6.4, should be 2.4.0.1, but nixpkgs doesn't have that
+      ghc864 = throw "static-haskell-nix: ghc863 is no longer supported, please upgrade";
       ghc865 = "Cabal_2_4_1_0"; # TODO this is technically incorrect for ghc 8.6.5, should be 2.4.0.1, but nixpkgs doesn't have that
+      ghc881 = "Cabal_3_0_0_0";
     }."${compiler}"),
 
   # Use `integer-simple` instead of `integer-gmp` to avoid linking in