Don't brand heap overflow as GHC bug

The "heap overflow" exception should not be branded as GHC bug.

I suggest it refer to the heap-controlling flags of the RTS instead: https://downloads.haskell.org/ghc/latest/docs/users_guide/runtime_control.html#rts-flag--M%20%E2%9F%A8size%E2%9F%A9

https://github.com/agda/agda/actions/runs/10506656146/job/29106737870?pr=7446#step:10:481

[428 of 435] Compiling Agda.Mimer.Mimer
panic! (the 'impossible' happened)
  GHC version 9.10.1:
	heap overflow

Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug

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