run: Don't inherit LD_PRELOAD/LD_AUDIT from the host

I don't think this env var makes much sense to pass into the sandbox
for similar reasons to LD_LIBRARY_PATH. Libraries from the host
just aren't relevant.

Users can still pass `--env=LD_PRELOAD=/foo` to use this functionality.

common/flatpak-run.c

@@ -518,10 +518,13 @@ typedef struct
 
 static const ExportData default_exports[] = {
  {"PATH", "/app/bin:/usr/bin"},
- /* We always want to unset LD_LIBRARY_PATH to avoid inheriting weird
- * dependencies from the host. But if not using ld.so.cache this is
- * later set. */
+ /* We always want to unset LD variables to avoid inheriting weird
+ * dependencies from the host. But if not using ld.so.cache LD_LIBRARY_PATH
+ is later set. */
  {"LD_LIBRARY_PATH", NULL},
+ {"LD_PRELOAD", NULL},
+ {"LD_AUDIT", NULL},
+
  {"XDG_CONFIG_DIRS", "/app/etc/xdg:/etc/xdg"},
  {"XDG_DATA_DIRS", "/app/share:/usr/share"},
  {"SHELL", "/bin/sh"},

doc/flatpak-run.xml

@@ -88,6 +88,8 @@
  <simplelist>
  <member>PATH</member>
  <member>LD_LIBRARY_PATH</member>
+ <member>LD_PRELOAD</member>
+ <member>LD_AUDIT</member>
  <member>XDG_CONFIG_DIRS</member>
  <member>XDG_DATA_DIRS</member>
  <member>XDG_RUNTIME_DIR</member>