fix(compute/serve): allow overriding of viceroy binary

[Integralist]

Fixes #828 #858

Problem

The compute serve subcommand requires Viceroy to be installed but some users have had problems with the Fastly CLI handling the downloading of the required Viceroy binary 'out-of-band'.

Examples of this are:

• When the CLI needs to be packaged alongside an external version of Viceroy (e.g. NixOS packaging).
• When the CLI is run inside Docker/CI and the network can be flaky (causing timeouts to be reached).

Solution

To solve this issue the user needs to be able to have the Fastly CLI use a version of the Viceroy binary that they have installed separately from the CLI (e.g. as part of a CI pipeline the user would manually install Viceroy, then install/run the Fastly CLI and inform it of where the Viceroy binary exists).

This PR introduces two new behaviours...

1. A new --viceroy-path flag.
2. A new FASTLY_VICEROY_USE_PATH environment variable.

--viceroy-path

A new --viceroy-path=<PATH_TO_VICEROY_BINARY> flag on the compute serve subcommand.

If set the Fastly CLI will use the version of the viceroy binary that the given path references.

If the --verbose flag is set, then the CLI will display the following INFO log output:

INFO: Using user provided install of Viceroy via --viceroy-path flag: /your/given/path/to/viceroy

FASTLY_VICEROY_USE_PATH

If the environment variable FASTLY_VICEROY_USE_PATH is set to either 1, true or TRUE then the CLI will attempt to lookup the viceroy binary from the user's $PATH.

FASTLY_VICEROY_USE_PATH=1 fastly compute serve

If passing the --verbose flag (and the binary exists in the user's PATH), then you'll see an INFO log indicating where the viceroy binary is being sourced from.