Fix compile error

[Procyonae]

…nslation, don't write required mutation string if we're about to overwrite it with incompatible bionics

Summary

None

Purpose of change

#79128 is preventing me compiling

In file included from src/units.h:19,
                 from src/game_constants.h:8,
                 from src/activity_type.h:9,
                 from src/activity_actor_definitions.h:13,
                 from src/iexamine.cpp:14:
src/iexamine.cpp: In function 'void iexamine::genemill(Character&, const tripoint_bub_ms&)':
src/iexamine.cpp:500:39: error: left operand of comma operator has no effect [-Werror=unused-value]
  500 |                 context_string = ( _( " %s" ), string_join( thresholds, ", " ) );
src/translations.h:89:20: note: in definition of macro '_'
   89 | #define _(STRING) (STRING)
      | 

Describe the solution

Fixes a couple of other bits with it while I were at it

Describe alternatives you've considered

Testing

Game compiles

Additional context

[akrieger]

I feel like that was supposed to originally be a string_format() call. But if you say so~

[akrieger]

But also like retitle the PR next time because you're fixing way more than a compile error here.