buildFHSEnv: symlink libexec into the FHS tree (#328685)

pkgs/build-support/build-fhsenv-bubblewrap/buildFHSEnv.nix

···
     ln -s /usr/lib32 $out/lib32
     ln -s /usr/lib64 $out/lib64
     ln -s /usr/lib64 $out/usr/lib
+    ln -s /usr/libexec $out/libexec
 
     # symlink 32-bit ld-linux so it's visible in /lib
     if [ -e $out/usr/lib32/ld-linux.so.2 ]; then

pkgs/build-support/build-fhsenv-bubblewrap/rootfs-builder/src/main.rs

···
 }
 
 fn remap_native_path(root: &Path, p: &Path) -> Option<PathBuf> {
-    if p.starts_with("bin/") || p.starts_with("sbin/") {
+    if p.starts_with("bin/") || p.starts_with("sbin/") || p.starts_with("libexec/") {
         return Some(PathBuf::from("usr/").join(p));
     }