Escape braces in assert expressions.

[Dirbaio]

Without this, defmt attempts to interpret the braces as format args, resulting in errors like

    defmt::assert!({1 + 1} == { 2 });

error: malformed format string (missing `:`)
  --> firmware/qemu/src/bin/assert.rs:10:5
   |
10 |     defmt::assert!({1 + 1} == { 2 });
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

[japaric]

Thanks for the fix!