Out of memory when building Agda 2.6.1 (and 2.6.2 on GHC 8.2)

[andreasabel]

GHC runs out of memory when trying to build recent versions of Agda.

E.g. https://matrix.hackage.haskell.org/#/package/Agda/2.6.2/ghc-8.2.2@1624083463 :

[363 of 397] Compiling Agda.TypeChecking.Serialise.Instances.Errors ( src/full/Agda/TypeChecking/Serialise/Instances/Errors.hs, /tmp/matrix-worker/1624117815/dist-newstyle/build/x86_64-linux/ghc-8.2.2/Agda-2.6.2/build/Agda/TypeChecking/Serialise/Instances/Errors.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.2.2 for x86_64-unknown-linux):
	heap overflow

<<ghc: 766726270256 bytes, 8724 GCs, 845318303/1804397512 avg/max bytes residency (73 samples), 1878M in use, 0.001 INIT (0.001 elapsed), 688.636 MUT (723.531 elapsed), 371.899 GC (371.043 elapsed) :ghc>>