fix(sccache): set TMPDIR within sccache cache directory

Configure sccache to use /nix/var/cache/sccache/tmp for temporary compilation artifacts. This ensures temp files are created within the sandbox-accessible path rather than in isolated build directories, preventing permission errors on Darwin while maintaining shared cache across builds.

Author: yvan-sraka

nix/cargo-pgrx/buildPgrxExtension.nix

@@ -172,6 +172,9 @@ let
           export RUSTC_WRAPPER="${sccache}/bin/sccache"
           export SCCACHE_DIR="/nix/var/cache/sccache"
           export SCCACHE_CACHE_SIZE="50G"
+          # Create temp directory for sccache within the cache directory
+          export TMPDIR="/nix/var/cache/sccache/tmp"
+          mkdir -p "$TMPDIR"
         else
           echo "sccache: cache directory not accessible in sandbox (needs extra-sandbox-paths configuration), skipping"
         fi