dirs-uap: handle VLC_LIBEXEC_PATH when VLC_PKG_LIBEXEC_DIR is requested

It's undocumented but was mapped the same way in e23555302f.

src/win32/dirs-uap.c

@@ -212,6 +212,9 @@ char *config_GetSysPath(vlc_sysdir_t type, const char *filename)
             dir = config_GetLibDir();
             break;
         case VLC_PKG_LIBEXEC_DIR:
+            dir = getenv ("VLC_LIBEXEC_PATH");
+            if (dir)
+                return strdup( dir );
             dir = config_GetLibexecDir();
             break;
         case VLC_SYSDATA_DIR: