nix-prefetch-git: provide fallback for $TMPDIR

$TMPDIR is not set by default in NixOS (nor Ubuntu), so without this
change nix-prefetch-git fails when using git-lfs.

pkgs/build-support/fetchgit/nix-prefetch-git

@@ -300,7 +300,9 @@ clone_user_rev() {
     local rev="${3:-HEAD}"
 
     if [ -n "$fetchLFS" ]; then
-        HOME=$TMPDIR
+        tmpHomePath="$(mktemp -d -p nix-prefetch-git-tmp-home-XXXXXXXXXX)"
+        exit_handlers+=(remove_tmpHomePath)
+        HOME="$tmpHomePath"
         git lfs install
     fi
 
@@ -408,6 +410,10 @@ remove_tmpPath() {
     rm -rf "$tmpPath"
 }
 
+remove_tmpHomePath() {
+    rm -rf "$tmpHomePath"
+}
+
 if test -n "$QUIET"; then
     quiet_mode
 fi