Add VOLTA_BYPASS override to short-circuit shim behavior

[charlespierce]

Closes #599

Info

• volta activate and volta deactivate are built as Shell functions that only work in specific unix shells.
• They also don't cleanly interact with the proposed update logic since they require a different install process than we want to support.
• [Updates] Remove 'volta activate' and 'volta deactivate' #562 Removed them as part of the updates work, which is still behind a feature flag for the moment.
• However, the escape hatch still has uses (most notably to work with custom versions of npm until we implement installing and pinning npm)
• To provide similar functionality, in a cross-platform way, we want to add a VOLTA_BYPASS environment variable that will cause the shims to skip their normal logic and shell out to the existing (non-Volta) system.

Changes

• Added logic to execute_tool that checks for the existence of VOLTA_BYPASS.
• If it is found, we short-circuit to a Passthrough command (i.e. using the system as if Volta didn't exist).
• We also add a custom error to the Passthrough command that will let the user know about VOLTA_BYPASS being enabled if the command fails to launch (e.g. if the requested tool doesn't exist on the system).

Tested

• Ran node --version with VOLTA_BYPASS set to verify that it ran the system node instead of the Volta node.
• Ran yarn --version with VOLTA_BYPASS set to verify that it shows the "Bypass enabled" error if it can't find a system executable.

[rwjblue]

Thank you for working on this!

[chriskrycho]

👏 ⚡️