grimblast: fixed image not going to buffer

[Frenagon]

Description of changes

Fixed an issue with my last commit, the screenshot was showing on the notification but it wasn't saved to the buffer, I double checked and this version now works as intended. I'm using the same tactic with tee used to show the screenshot when using copysave

Things done

• For new programs

  • [ ] Add the program to the README table, with yourself as the maintainer
  • [ ] Add a README for the program itself
  • [ ] Add Makefile (and Nix derivation optionally, otherwise @fufexan will do it)
  • [ ] If the program is a script, add it to the CI checks matrix

• For changes

  • [ ] Add changes to the CHANGELOG