refactor(3p/nixpkgs): Pin nixpkgs sources via niv

There's no need for us to reinvent the wheel here.

niv pins are in //third_party/sources, and niv commands need to be run
with `niv -s third_party/sources/sources.json` to operate on the
correct file.

Note that niv by default wants to put the loader file in
//nix/sources.nix. This file has been moved to
//third_party/sources/default.nix which makes niv throw a warning, but
everything still works as intended.

Change-Id: I2b2a6f8edf33c429a6d7be9d174ba1996d9a0193
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5143
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>

third_party/sources/sources.json

@@ -0,0 +1,26 @@
+{
+    "nixpkgs": {
+        "branch": "nixos-unstable",
+        "description": "Nix Packages collection",
+        "homepage": "",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "efeefb2af1469a5d1f0ae7ca8f0dfd9bb87d5cfb",
+        "sha256": "1b3sxslv5id61phq7zx3lybw72x29bx9595i8m708fax7iml07j2",
+        "type": "tarball",
+        "url": "https://github.com/NixOS/nixpkgs/archive/efeefb2af1469a5d1f0ae7ca8f0dfd9bb87d5cfb.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
+    "nixpkgs-stable": {
+        "branch": "nixos-21.11",
+        "description": "Nix Packages collection",
+        "homepage": "",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "0f316e4d72daed659233817ffe52bf08e081b5de",
+        "sha256": "0vh0fk5is5s9l0lxpi16aabv2kk1fwklr7szy731kfcz9gdrr65l",
+        "type": "tarball",
+        "url": "https://github.com/NixOS/nixpkgs/archive/0f316e4d72daed659233817ffe52bf08e081b5de.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    }
+}