Revert "nixpkgs: pin to unstable and unpin pre-commit"

This reverts commit bf4b11f.

The latest nixpkgs version still breakes things.

Author: David Arnold

nix/sources.json

@@ -36,15 +36,27 @@
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
     "nixpkgs": {
-        "branch": "nixpkgs-unstable",
-        "description": "Nix Packages collection",
-        "homepage": null,
+        "branch": "nixos-20.03",
+        "description": "A read-only mirror of NixOS/nixpkgs tracking the released channels. Send issues and PRs to",
+        "homepage": "https://github.com/NixOS/nixpkgs",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "2d6cbbe4627f6fe4a179c681537b0a3e4f59b732",
-        "sha256": "12kh498rfr0yscvzidbgvip3x2zah098nsi6mjj3vfrlz2aximi5",
+        "rev": "99a3d7a86fce9e9c9f23b3e304d7d2b1270a12b8",
+        "sha256": "0i40cl3n6600z2lkwrpiy28dcnv2r63fcgfswj91aaf1xfn2chql",
+        "type": "tarball",
+        "url": "https://github.com/NixOS/nixpkgs/archive/99a3d7a86fce9e9c9f23b3e304d7d2b1270a12b8.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
+    "pre-commit": {
+        "branch": "v2.6.0",
+        "description": "A framework for managing and maintaining multi-language pre-commit hooks.",
+        "homepage": "https://pre-commit.com",
+        "owner": "pre-commit",
+        "repo": "pre-commit",
+        "rev": "6fe1702ee106f4d7f4a8ad73550db2145208ef24",
+        "sha256": "144hcnz8vz07nkx7hk8a3ac822186ardwxa8jnl6s8qvm5ip92f2",
         "type": "tarball",
-        "url": "https://github.com/NixOS/nixpkgs/archive/2d6cbbe4627f6fe4a179c681537b0a3e4f59b732.tar.gz",
+        "url": "https://github.com/pre-commit/pre-commit/archive/6fe1702ee106f4d7f4a8ad73550db2145208ef24.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     }
 }