More defined behaviour for `tenv tf install` command

[Nmishin]

tenv tf works as a proxy for terraform or opentofu depending on the version files present in project.
https://github.com/tofuutils/tenv?tab=readme-ov-file#project-binaries

It can lead hardly predicted behaviour when terraform or opentofu binary will be installed.

I think that tenv tf should be pointed directly to the terraform.

[dvaumoron]

There is already a way todo that : #339

[dvaumoron]

wait, I am wrong, you want to disable tf as agnostic ? you can call the terraform proxy for that.

tf was added for #73

[Nmishin]

OK, I agree. It doesn't make sense to change the current behavior (especially since we haven't receiving many issue requests about it). We just need to update or rewrite the README to make that behavior clearer.

Will do this, as a part of this issues.