Update Etcher binaries to 1.7.8

Merge request:

• Download binaries from https://www.balena.io/etcher/
  • Windows 64-bit portable
  • macOS
• Copy the IMG and the etcher-binary directory to 2 USB sticks
• Download the binaries 1 more time with torsocks --isolate
  • Compare downloads using sha256sum
• Test macOS install
  • Install from macOS
  • Start from macOS install
• Test Windows install
  • Install from Windows
  • Start from Windows install
• Copy binaries to /etcher
• Commit and push

Review:

• Download binaries from https://www.balena.io/etcher/
  • Windows 64-bit portable
  • macOS
• Compare downloads using sha256sum
• Merge and push
• Test download:
  • https://tails.boum.org/etcher/Etcher-Portable.exe
  • https://tails.boum.org/etcher/Etcher.dmg
  • Compare downloads using sha256sum

NB: Disregard the name of the branch. I tested with 1.7.8.

Also, tails!778 (merged) should be merged after this MR is merged.