fix: remove TMPDIR override that breaks mktemp in cargo-pgrx

Setting TMPDIR globally breaks mktemp calls in preBuildAndTest phase.
Let sccache use default temp directory locations while still using
/nix/var/cache/sccache for actual cache storage.

Author: yvan-sraka

nix/cargo-pgrx/buildPgrxExtension.nix

@@ -172,9 +172,6 @@ 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