force-inline forceValue

forceValue is extremely hot. interestingly adding likeliness annotations
to the branches does not seem to make a difference.

before:

  Time (mean ± σ):      4.224 s ±  0.005 s    [User: 3.711 s, System: 0.512 s]
  Range (min … max):    4.218 s …  4.234 s    10 runs

after:

  Time (mean ± σ):      4.140 s ±  0.009 s    [User: 3.647 s, System: 0.492 s]
  Range (min … max):    4.130 s …  4.152 s    10 runs

src/libexpr/eval-inline.hh

@@ -81,6 +81,7 @@ Env & EvalState::allocEnv(size_t size)
 }
 
 
+[[gnu::always_inline]]
 void EvalState::forceValue(Value & v, const PosIdx pos)
 {
     if (v.isThunk()) {