remove usage of IFD in nix packages

Lix builders have IFD disabled, and the changes needed to support this
are minimal.

We do need to do slightly more manual work to keep rust-toolchain.toml
in sync with flake.nix, but it's not significant.

flake.lock

@@ -226,7 +226,8 @@
         "flake-compat": "flake-compat_2",
         "flake-utils": "flake-utils_2",
         "nix-filter": "nix-filter",
-        "nixpkgs": "nixpkgs_2"
+        "nixpkgs": "nixpkgs_2",
+        "rust-manifest": "rust-manifest"
       }
     },
     "rust-analyzer-src": {
@@ -246,6 +247,18 @@
         "type": "github"
       }
     },
+    "rust-manifest": {
+      "flake": false,
+      "locked": {
+        "narHash": "sha256-aZFye4UrtlcvLHrISldx4g9uGt3thDbVlLMK5keBSj0=",
+        "type": "file",
+        "url": "https://static.rust-lang.org/dist/channel-rust-1.78.0.toml"
+      },
+      "original": {
+        "type": "file",
+        "url": "https://static.rust-lang.org/dist/channel-rust-1.78.0.toml"
+      }
+    },
     "systems": {
       "locked": {
         "lastModified": 1681028828,