zig_0_9: build with baseline CPU target

26b9a2f4a1 changes Zig 0.10 to build the
compiler (notably *not* its outputs, at least not by default) with
its baseline CPU target, but we should ideally do it for both versions
to increase reproducibility, as well as increase the number of users who
are able to use Hydra-provided Zig binaries.

This also adds a comment above the flag in 0.10, to explain why we're adding
the flag, as we do with the RPATH one.

See https://github.com/NixOS/nixpkgs/issues/214356 and https://github.com/NixOS/nixpkgs/issues/185665
for further context.

pkgs/development/compilers/zig/0.10.nix

@@ -47,6 +47,8 @@ stdenv.mkDerivation rec {
   cmakeFlags = [
     # file RPATH_CHANGE could not write new RPATH
     "-DCMAKE_SKIP_BUILD_RPATH=ON"
+
+    # ensure determinism in the compiler build
     "-DZIG_TARGET_MCPU=baseline"
   ];
 

pkgs/development/compilers/zig/0.9.1.nix

@@ -62,6 +62,9 @@ stdenv.mkDerivation rec {
   cmakeFlags = [
     # file RPATH_CHANGE could not write new RPATH
     "-DCMAKE_SKIP_BUILD_RPATH=ON"
+
+    # ensure determinism in the compiler build
+    "-DZIG_TARGET_MCPU=baseline"
   ];
 
   doCheck = true;