only set `CONDUIT_VERSION_EXTRA` for final build

This prevents us from needing to recompile the dependencies when that
environment variable changes, which generally changes on every commit,
which is far more frequently than the dependencies are actually changed.

nix/pkgs/default/default.nix

@@ -14,15 +14,13 @@
 }:
 
 let
-  env =
+  buildDepsOnlyEnv =
     let
       rocksdb' = rocksdb.override {
         enableJemalloc = builtins.elem "jemalloc" features;
       };
     in
     {
-      CONDUIT_VERSION_EXTRA =
-        inputs.self.shortRev or inputs.self.dirtyShortRev;
       ROCKSDB_INCLUDE_DIR = "${rocksdb'}/include";
       ROCKSDB_LIB_DIR = "${rocksdb'}/lib";
     }
@@ -35,26 +33,45 @@ let
         rust
         stdenv;
     });
-in
 
-craneLib.buildPackage rec {
-  inherit
-    (craneLib.crateNameFromCargoToml {
-      cargoToml = "${inputs.self}/Cargo.toml";
-    })
-    pname
-    version;
+  buildPackageEnv = {
+    CONDUIT_VERSION_EXTRA = inputs.self.shortRev or inputs.self.dirtyShortRev;
+  } // buildDepsOnlyEnv;
+
+  commonAttrs = {
+    inherit
+      (craneLib.crateNameFromCargoToml {
+        cargoToml = "${inputs.self}/Cargo.toml";
+      })
+      pname
+      version;
 
-  src = let filter = inputs.nix-filter.lib; in filter {
-    root = inputs.self;
+    src = let filter = inputs.nix-filter.lib; in filter {
+      root = inputs.self;
 
-    # Keep sorted
-    include = [
-      "Cargo.lock"
-      "Cargo.toml"
-      "src"
+      # Keep sorted
+      include = [
+        "Cargo.lock"
+        "Cargo.toml"
+        "src"
+      ];
+    };
+
+    nativeBuildInputs = [
+      # bindgen needs the build platform's libclang. Apparently due to "splicing
+      # weirdness", pkgs.rustPlatform.bindgenHook on its own doesn't quite do the
+      # right thing here.
+      pkgsBuildHost.rustPlatform.bindgenHook
     ];
+
+    CARGO_PROFILE = profile;
   };
+in
+
+craneLib.buildPackage ( commonAttrs // {
+  cargoArtifacts = craneLib.buildDepsOnly (commonAttrs // {
+    env = buildDepsOnlyEnv;
+  });
 
   cargoExtraArgs = "--locked "
     + lib.optionalString
@@ -67,20 +84,11 @@ craneLib.buildPackage rec {
   # This is redundant with CI
   doCheck = false;
 
-  nativeBuildInputs = [
-    # bindgen needs the build platform's libclang. Apparently due to "splicing
-    # weirdness", pkgs.rustPlatform.bindgenHook on its own doesn't quite do the
-    # right thing here.
-    pkgsBuildHost.rustPlatform.bindgenHook
-  ];
-
-  CARGO_PROFILE = profile;
-
-  inherit env;
+  env = buildPackageEnv;
 
   passthru = {
-    inherit env;
+    env = buildPackageEnv;
   };
 
-  meta.mainProgram = pname;
-}
+  meta.mainProgram = commonAttrs.pname;
+})