Set kern.curproc_arch_affinity=0 to escape Rosetta

By default, once you enter x86_64 Rosetta 2, macOS will try to run
everything in x86_64. So an x86_64 Nix will still try to use x86_64
even when system = aarch64-darwin. To avoid this we can set
kern.curproc_arch_affinity sysctl. With kern.curproc_arch_affinity=0,
we ignore this preference.

This is based on how
https://opensource.apple.com/source/system_cmds/system_cmds-880.40.5/arch.tproj/arch.c.auto.html
works. Completely undocumented, but seems to work!

Note, you can verify this works with this impure Nix expression:

```
  {
    a = derivation {
      name = "a";
      system = "aarch64-darwin";
      builder = "/bin/sh";
      args = [ "-e" (builtins.toFile "builder" ''
        [ "$(/usr/bin/arch)" = arm64 ]
        [ "$(/usr/bin/arch -arch x86_64 /bin/sh -c /usr/bin/arch)" = i386 ]
        [ "$(/usr/bin/arch -arch arm64 /bin/sh -c /usr/bin/arch)" = arm64 ]
        /usr/bin/touch $out
      '') ];
    };

    b = derivation {
      name = "b";
      system = "x86_64-darwin";
      builder = "/bin/sh";
      args = [ "-e" (builtins.toFile "builder" ''
        [ "$(/usr/bin/arch)" = i386 ]
        [ "$(/usr/bin/arch -arch x86_64 /bin/sh -c /usr/bin/arch)" = i386 ]
        [ "$(/usr/bin/arch -arch arm64 /bin/sh -c /usr/bin/arch)" = arm64 ]
        /usr/bin/touch $out
      '') ];
    };
  }
```

1 files changed, 5 insertions, 0 deletions

diff --git a/src/libstore/build/derivation-goal.cc b/src/libstore/build/derivation-goal.cc
index 35f365795..a02ddb950 100644
--- a/src/libstore/build/derivation-goal.cc
+++ b/src/libstore/build/derivation-goal.cc
@@ -52,6 +52,7 @@
 
 #if __APPLE__
 #include <spawn.h>
+#include <sys/sysctl.h>
 #endif
 
 #include <pwd.h>
@@ -2869,6 +2870,10 @@ void DerivationGoal::runChild()
             throw SysError("failed to initialize builder");
 
         if (drv->platform == "aarch64-darwin") {
+            // Unset kern.curproc_arch_affinity so we can escape Rosetta
+            int affinity = 0;
+            sysctlbyname("kern.curproc_arch_affinity", NULL, NULL, &affinity, sizeof(affinity));
+
             cpu_type_t cpu = CPU_TYPE_ARM64;
             posix_spawnattr_setbinpref_np(&attrp, 1, &cpu, NULL);
         } else if (drv->platform == "x86_64-darwin") {