Bump nixpkgs to 21.11

Current nixpkgs doesn't support macOS Big Sur (11). Its support landed in
nixpkgs in 21.11, but that also broke support for Catalina (10.15), so
switch CI workflow to use "macos-11" runner.

Fixes tweag#175

.github/workflows/workflow.yaml

@@ -6,7 +6,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-10.15] # TODO: this job does not yet work on more recent macOS (macos-11)
+        os: 
+          - ubuntu-latest
+          - macos-11
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2.4.0
@@ -32,7 +34,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest] # TODO: this job does not yet work on any version of macOS
+        os: 
+          - ubuntu-latest # TODO: this job does not yet work on any version of macOS
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2.4.0

WORKSPACE

@@ -41,8 +41,8 @@ bazel_skylib_workspace()
 nixpkgs_git_repository(
     name = "remote_nixpkgs",
     remote = "https://github.com/NixOS/nixpkgs",
-    revision = "18.09",
-    sha256 = "6451af4083485e13daa427f745cbf859bc23cb8b70454c017887c006a13bd65e",
+    revision = "21.11",
+    sha256 = "c77bb41cf5dd82f4718fa789d49363f512bb6fa6bc25f8d60902fe2d698ed7cc",
 )
 
 nixpkgs_local_repository(

nixpkgs.json

@@ -2,6 +2,6 @@
   "owner": "NixOS",
   "repo": "nixpkgs",
   "branch": "nixpkgs-unstable",
-  "rev": "ad4db3f4d8ae54482c63c31c14921cb73953548d",
-  "sha256": "1fwl898f6wznkjpwq11brgadz6iff5w5f4lwj2l7ax2rz7r03mnn"
+  "rev": "506445d88e183bce80e47fc612c710eb592045ed",
+  "sha256": "162dywda2dvfj1248afxc45kcrg83appjd0nmdb541hl7rnncf02"
 }