fix: enable sccache on Darwin with isolated temp directory

Use SCCACHE_NO_DAEMON=1 and redirect TMPDIR to sccache cache dir
to avoid cross-sandbox permission errors on Darwin

Author: yvan-sraka

nix/cargo-pgrx/buildPgrxExtension.nix

@@ -165,21 +165,26 @@ let
     buildPhase = ''
       runHook preBuild
 
-      ${lib.optionalString (!stdenv.hostPlatform.isDarwin) ''
-        if [[ -d "/nix/var/cache/sccache" && -w "/nix/var/cache/sccache" ]]; then
-          # Test if sccache can actually write to the directory (sandbox check)
-          if touch "/nix/var/cache/sccache/.test" 2>/dev/null && rm -f "/nix/var/cache/sccache/.test" 2>/dev/null; then
-            echo "sccache: cache directory available and writable in sandbox, enabling"
-            export RUSTC_WRAPPER="${sccache}/bin/sccache"
-            export SCCACHE_DIR="/nix/var/cache/sccache"
-            export SCCACHE_CACHE_SIZE="50G"
-          else
-            echo "sccache: cache directory not accessible in sandbox (needs extra-sandbox-paths configuration), skipping"
-          fi
+      if [[ -d "/nix/var/cache/sccache" && -w "/nix/var/cache/sccache" ]]; then
+        # Test if sccache can actually write to the directory (sandbox check)
+        if touch "/nix/var/cache/sccache/.test" 2>/dev/null && rm -f "/nix/var/cache/sccache/.test" 2>/dev/null; then
+          echo "sccache: cache directory available and writable in sandbox, enabling"
+          export RUSTC_WRAPPER="${sccache}/bin/sccache"
+          export SCCACHE_DIR="/nix/var/cache/sccache"
+          export SCCACHE_CACHE_SIZE="50G"
+          ${lib.optionalString stdenv.hostPlatform.isDarwin ''
+            # On Darwin, sccache daemon mode causes cross-sandbox permission issues
+            # Use no-daemon and redirect temp files to shared cache directory
+            export SCCACHE_NO_DAEMON=1
+            export TMPDIR="/nix/var/cache/sccache/tmp"
+            mkdir -p "$TMPDIR"
+          ''}
         else
-          echo "sccache: cache directory not available, skipping"
+          echo "sccache: cache directory not accessible in sandbox (needs extra-sandbox-paths configuration), skipping"
         fi
-      ''}
+      else
+        echo "sccache: cache directory not available, skipping"
+      fi
 
       echo "Executing cargo-pgrx buildPhase"
       ${preBuildAndTest}