archive-builds.sh: Ignore non-directories in /package/pkg

Some .have_archived_on_tape plain files have appeared in /package/pkg so
this script fails when it tries to change the current working directory
into a path which is not a directory.

Check that the path is a directory before changing into it. This can be
done with the existing check, because a check for a subdirectory implies
that the parent is a directory, too. However, we need to use absolute
pathnames, so modify the wildcard to produce absolute pathnames.

tools/archive-build.sh

@@ -7,12 +7,11 @@ if [ "$(hostname -s)" != "ellie" ]; then
     exit 1
 fi
 
-cd /package/pkg
 umask 022
 
-for d in *; do
-    cd /package/pkg/$d
-    if [[ -d build && $(stat --format=%U .) == "bin" ]]; then
+for d in /package/pkg/*; do
+    if [[ -d $d/build && $(stat --format=%U $d) == "bin" ]]; then
+        cd $d
         echo "archive $d/build"
         tar --create --zstd --file build.tar.zstd build/ && rm -rf build
     fi