feat(Electron 8.x upgrade): Include electron version in build pipeline variable names

[DaveTryon]

Description of changes

This replaces the ELECTRON_MIRROR_VAR and ELECTRON_CUSTOM_DIR_VAR environment variables with ELECTRON_701_MIRROR_VAR and ELECTRON_701_CUSTOM_DIR_VAR, respectively. We're doing this to allow the build pipeline to support concurrent versions of Electron as we migrate to 8.x. A similar strategy will be applied to future version migrations.

Note that ELECTRON_MIRROR and ELECTRON_CUSTOM_DIR are intentionally not changed, since these are the names of the data passed to the build tools.

Unsigned build 2020.422.21 has been queued to validate that the correct version of Electron is getting picked up by the build pipeline.

Pull request checklist

• Addresses an existing issue: 1710649
• Ran yarn fastpass
• [ n/a ] Added/updated relevant unit test(s) (and ran yarn test)
• Verified code coverage for the changes made. Check coverage report at: <rootDir>/test-results/unit/coverage
• PR title AND final merge commit title both start with a semantic tag (fix:, chore:, feat(feature-name):, refactor:). See CONTRIBUTING.md.
• (UI changes only) Added screenshots/GIFs to description above
• (UI changes only) Verified usability with NVDA/JAWS