scripts: Fix a stack use-after-free

I think this changed in a recent refactoring; basically since
we're passing this stack-allocated value to the child spawn
function we need to keep it alive.  This of course would
have been caught by Rust...

```
==672376==ERROR: AddressSanitizer: stack-use-after-scope on address 0x7ffc290d9440 at pc 0x55c88c318946 bp 0x7ffc290d8b10 sp 0x7ffc290d8b08
    #0 0x55c88c318945 in script_child_setup src/libpriv/rpmostree-scripts.cxx:272
    #1 0x7f92089da902  (/lib64/libglib-2.0.so.0+0x9f902)
    #2 0x7f92089de20f  (/lib64/libglib-2.0.so.0+0xa320f)
    #3 0x7f92089de52e  (/lib64/libglib-2.0.so.0+0xa352e)
    #4 0x7f92089def02 in g_spawn_async_with_pipes (/lib64/libglib-2.0.so.0+0xa3f02)
    #5 0x7f9208b7445f  (/lib64/libgio-2.0.so.0+0xab45f)
    #6 0x7f9208b736d8 in g_subprocess_launcher_spawnv (/lib64/libgio-2.0.so.0+0xaa6d8)
    #7 0x55c88c3831b9 in rpmostree_bwrap_execute src/libpriv/rpmostree-bwrap.cxx:504
    #8 0x55c88c3836df in rpmostree_bwrap_run_captured src/libpriv/rpmostree-bwrap.cxx:450
    #9 0x55c88c31b5f1 in rpmostree_run_script_in_bwrap_container src/libpriv/rpmostree-scripts.cxx:469
    #10 0x55c88c31ca9d in impl_run_rpm_script src/libpriv/rpmostree-scripts.cxx:588
    #11 0x55c88c31d22b in run_script src/libpriv/rpmostree-scripts.cxx:637
    #12 0x55c88c31d22b in rpmostree_script_run_sync src/libpriv/rpmostree-scripts.cxx:778
    #13 0x55c88c2ef830 in run_script_sync src/libpriv/rpmostree-core.cxx:3661
    #14 0x55c88c30afa6 in rpmostree_context_assemble src/libpriv/rpmostree-core.cxx:4422
    #15 0x55c88c34a9af in install_packages src/app/rpmostree-compose-builtin-tree.cxx:451
    #16 0x55c88c34c174 in impl_install_tree src/app/rpmostree-compose-builtin-tree.cxx:925
    #17 0x55c88c350f84 in rpmostree_compose_builtin_tree src/app/rpmostree-compose-builtin-tree.cxx:1421
    #18 0x55c88c276ec8 in rpmostree_handle_subcommand src/app/libmain.cxx:405
    #19 0x55c88c27827c in rpmostree_main_inner src/app/libmain.cxx:521
    #20 0x55c88c27827c in rpmostreecxx::rpmostree_main(rust::cxxbridge1::Slice<rust::cxxbridge1::Str const>) src/app/libmain.cxx:546
    #21 0x55c88c271c25 in operator() /var/srv/walters/src/github/coreos/rpm-ostree/rpmostree-cxxrs.cxx:1257
    #22 0x55c88c271c25 in trycatch<rpmostreecxx::rpmostreecxx$cxxbridge1$rpmostree_main(rust::cxxbridge1::Slice<const rust::cxxbridge1::Str>)::<lambda()>, rpmostreecxx::rpmostreecxx$cxxbridge1$rpmostree_main(rust::cxxbridge1::Slice<const rust::cxxbridge1::Str>)::<lambda(char const*)> > /var/srv/walters/src/github/coreos/rpm-ostree/rpmostree-cxxrs.cxx:997
    #23 0x55c88c271c25 in rpmostreecxx$cxxbridge1$rpmostree_main /var/srv/walters/src/github/coreos/rpm-ostree/rpmostree-cxxrs.cxx:1255
    #24 0x55c88c0468f7 in rpmostree_rust::ffi::rpmostree_main::hfedda48c684245ce rust/src/lib.rs:25
    #25 0x55c88c0468f7 in rpm_ostree::inner_main::hf078b99ca4b270aa rust/src/main.rs:9
    #26 0x55c88c0468f7 in rpm_ostree::main::hc0ca527cfaa3f556 rust/src/main.rs:28
    #27 0x55c88c046b22 in core::ops::function::FnOnce::call_once::h8567110dac55274e /var/home/walters/.rustup/toolchains/1.48-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:227
    #28 0x55c88c046b22 in std::sys_common::backtrace::__rust_begin_short_backtrace::h1c67f2f52d05cfa0 /var/home/walters/.rustup/toolchains/1.48-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys_common/backtrace.rs:137
    #29 0x55c88c045fd7 in main (/usr/bin/rpm-ostree+0xc9fd7)
    #30 0x7f92076091e1 in __libc_start_main (/lib64/libc.so.6+0x281e1)
    #31 0x55c88c045b9d in _start (/usr/bin/rpm-ostree+0xc9b9d)

Address 0x7ffc290d9440 is located in stack of thread T0 at offset 272 in frame
    #0 0x55c88c31a1af in rpmostree_run_script_in_bwrap_container src/libpriv/rpmostree-scripts.cxx:349
```

src/libpriv/rpmostree-scripts.cxx

@@ -401,6 +401,9 @@ rpmostree_run_script_in_bwrap_container (int rootfs_fd,
 
   glnx_autofd int stdout_fd = -1;
   glnx_autofd int stderr_fd = -1;
+  struct ChildSetupData data = { .stdin_fd = stdin_fd,
+                                 .stdout_fd = -1,
+                                 .stderr_fd = -1, };
   GLnxTmpfile buffered_output = { 0, };
   const char *id = glnx_strjoina ("rpm-ostree(", pkg_script, ")");
   if (debugging_script || stdin_fd == STDIN_FILENO)
@@ -410,9 +413,6 @@ rpmostree_run_script_in_bwrap_container (int rootfs_fd,
     }
   else
     {
-      struct ChildSetupData data = { .stdin_fd = stdin_fd,
-                                     .stdout_fd = -1,
-                                     .stderr_fd = -1, };
 
       rust::Slice<const uint8_t> scriptslice{(guint8*)script, strlen (script)};
       glnx_fd_close int script_memfd = rpmostreecxx::sealed_memfd (pkg_script, scriptslice);