fix: include v8 flags used by electron in ffmpeg build

[jkleinsc]

Description of Change

mksnapshot comes from the ffmpeg build and needs the same v8 flags used for the static library build.

Right now without those flags the mksnapshot we are distributing for 3-1-x ends up causing segfaults if you use Promises in the snapshot script. See https://github.com/electron/electron/issues/18420 for more details

Checklist

• [x] PR description included and stakeholders cc'd
• [x] script/update runs without error
• [x] PR title follows semantic commit guidelines