electron*: refactor 4+ to common expression, add 7, updates

* print-hashes.nix uses upstream-provided SHA256SUMS file
* electron_4: 4.2.8 -> 4.2.12
* electron_5: 5.0.8 -> 5.0.13
* electron_6: 6.0.1 -> 6.1.7
* electron_7:: init at 7.1.10

FWIW:
electron_4 previously did not include at-spi2-core, only at-spi2-atk.
Both are now included, matching other versions for a slightly simpler
expression.