Don't capture local-scoped faces in prompt

ASan shows that we resolve a face spec owned by a freed stack variable.

    =================================================================
    ==2263300==ERROR: AddressSanitizer: stack-use-after-return on address 0x7a9316c33918 at pc 0x633ea421d8ea bp 0x7ffca001e980 sp 0x7ffca001e970
    READ of size 8 at 0x7a9316c33918 thread T0
        ...
        #6 0x633ea421d8e9 in Kakoune::FaceRegistry::resolve_spec(Kakoune::FaceSpec const&) const src/face_registry.cc:128
        ...

    Address 0x7a9316c33918 is located in stack of thread T0 at offset 2328 in frame
        #0 0x633ea427a095 in operator() src/commands.cc:2267

      This frame has 26 object(s):
        [32, 36) '<unknown>'
        ...
        [544, 560) 'disable_hooks' (line 2269)
        ...
        [928, 2432) 'local_scope' (line 2271) <== Memory access at offset 2328 is inside this variable

0 files changed, 0 insertions, 0 deletions