bintools-wrapper: look only at shared libraries in library directories

To decide whether a store library directory belongs in the rpath,
ld-wrapper looks up each entry of the directory among the libraries
being linked.  But those are all named lib*.so (from -l), or *.so or
*.so.* (when given by path), so only entries matching *.so* can be
among them.  List only those, leaving bash with far fewer entries to
look up; the directory is still read just once.

Linking an empty C program with 1500 different store library
directories, which together have 12241 entries but only 7614 matching
*.so*, now takes about 0.40 s instead of 0.44 s, in a VM where starting
a process is slow.

Assisted-by: Claude Code:claude-opus-5-5
Signed-off-by: Anders Kaseorg <andersk@mit.edu>

pkgs/build-support/bintools-wrapper/ld-wrapper.sh

@@ -221,7 +221,9 @@ if [[ "$NIX_DONT_SET_RPATH_@suffixSalt@" != 1 && "$linkType" != static-pie ]]; t
         if [ -n "${rpaths[$dir]:-}" ] || [[ "$dir" != "${NIX_STORE:-}"/* ]]; then
             continue
         fi
-        for path in "$dir"/*; do
+        # Every name in libs is that of a shared library, ending in .so or
+        # containing .so., so the rest of the directory can be skipped.
+        for path in "$dir"/*.so*; do
             file="${path##*/}"
             if [ "${libs[$file]:-}" ]; then
                 # This library may have been provided by a previous directory,