We support --env in compute serve, which acts as a switch for the fastly.toml manifest that is loaded.

For example, fastly compute serve --env stage will attempt to use a fastly.stage.toml manifest file.

Customers have requested the same support for --env with the compute deploy command.

This PR modifies the following compute commands because it made sense to add support for --env to them...

• compute build
• compute deploy
• compute publish
• compute hashsum
• compute hash-files

I've manually tested these commands to ensure they're working as intended.

NOTE: The other reason for modifying hashsum and hash-files is that I realised if we support modifying a build to use --env then it means we need to be able to reproduce that build for the hashing commands.