Fix determinism by defaulting codegenUnits to 1, not NIX_BUILD_CORES

pkgs/build-support/rust/build-rust-crate/build-crate.nix

@@ -12,7 +12,7 @@
     baseRustcOpts =
       [
         (if release then "-C opt-level=3" else "-C debuginfo=2")
-        "-C codegen-units=${codegenUnits}"
+        "-C codegen-units=${toString codegenUnits}"
         "--remap-path-prefix=$NIX_BUILD_TOP=/"
         (mkRustcDepArgs dependencies crateRenames)
         (mkRustcFeatureArgs crateFeatures)

pkgs/build-support/rust/build-rust-crate/configure-crate.nix

@@ -25,7 +25,7 @@ let version_ = lib.splitString "-" crateVersion;
     version = lib.splitVersion (lib.head version_);
     rustcOpts = lib.foldl' (opts: opt: opts + " " + opt)
         (if release then "-C opt-level=3" else "-C debuginfo=2")
-        (["-C codegen-units=${codegenUnits}"] ++ extraRustcOptsForBuildRs);
+        (["-C codegen-units=${toString codegenUnits}"] ++ extraRustcOptsForBuildRs);
     buildDeps = mkRustcDepArgs buildDependencies crateRenames;
     authors = lib.concatStringsSep ":" crateAuthors;
     optLevel = if release then 3 else 0;

pkgs/build-support/rust/build-rust-crate/default.nix

@@ -316,7 +316,7 @@ crate_: lib.makeOverridable
       colors = lib.attrByPath [ "colors" ] "always" crate;
       extraLinkFlags = lib.concatStringsSep " " (crate.extraLinkFlags or [ ]);
       edition = crate.edition or null;
-      codegenUnits = if crate ? codegenUnits then crate.codegenUnits else "$NIX_BUILD_CORES";
+      codegenUnits = if crate ? codegenUnits then crate.codegenUnits else 1;
       extraRustcOpts =
         lib.optionals (crate ? extraRustcOpts) crate.extraRustcOpts
           ++ extraRustcOpts_