fix(envbuilder.go)!: rename MagicDirBase CLI option

[SasSwart]

This PR is a breaking change!
It renames a publicly accessible CLI option referenced in #184.
It should not be merged into release/1.0 as this would break at least the terraform provider that relies on this option.

This commit may be cherry-picked into release/2.0 when that branch is created.

This PR closes #184.

[johnstcn]

@mafredri Are you happy with us merging this to main now? Or would you like to defer this to a later date so we don't end up needing to do too many backport changes?

[mafredri]

@mafredri Are you happy with us merging this to main now? Or would you like to defer this to a later date so we don't end up needing to do too many backport changes?

(We could just make it backwards compatible for a period of time by including the old property (and checking it too) but marking it deprecated. That way we’re not in a rush to sync the changes to the provider.)

I’m also fine with merging as-is, but I’d avoid cherry-picking it into release/1.0 unless there are conflicts. Our next non-patch release should be release/1.1, which we branch out from main, and I think it’s fine to include this there.