Added unzip to check_dependencies

[eliaskickbush]

Description

This PR adds a check for existence of the unzip binary in check_dependencies to fail early when unzip is not present and give a more explicit error message.

While it's uncommon for systems to not have unzip present, a fresh installation of WSL does not have unzip, at least in my distribution. The current behavior goes as follows:

user@computer:~/Projects/tfenv$ tfenv install 1.15.2
tfenv: Installing Terraform v1.15.2
tfenv: Downloading release tarball from https://releases.hashicorp.com/terraform/1.15.2/terraform_1.15.2_linux_amd64.zip
################################################################################################################################################################################################################################## 100.0%
tfenv: Downloading SHA hash file from https://releases.hashicorp.com/terraform/1.15.2/terraform_1.15.2_SHA256SUMS
tfenv: Not instructed to use Local PGP (/home/elias/.tfenv/use-{gpgv,gnupg}) & No keybase install found, skipping OpenPGP signature verification
tfenv: SHA256 hash matched!
/home/elias/.tfenv/libexec/tfenv-install: line 403: unzip: command not found
tfenv: [ERROR] Tarball unzip failed

With this change, a check runs before attempting to unzip (by the already present check_dependencies). The new user experience is:

user@computer:~/Projects/tfenv$ ./bin/tfenv install 1.15.2
tfenv: [ERROR] unzip is a requirement and your device does not have it. Install unzip using your preferred method.
tfenv: [ERROR] Failed to resolve 1.15.2 version

command is present on sh, so it should be usable in BSD systems. I have however not checked in an actual BSD-based system.

Issue Link

I have not created an issue for this PR.

Testing

• Ran ./test/run.sh locally and all tests pass
• Tested on at least one platform (note which below)
• New tests added for new functionality (if applicable)

Platform tested:

Quality Checklist

• Shell quoting verified — all variables quoted, braces used
• Cross-platform considered — BSD vs GNU tool differences
• No unintended changes to other commands
• README.md updated (if user-facing change)
• CHANGELOG.md updated (if notable change)

Disclaimer

This is my first contribution to this project, sorry if something is off standard, I tried to follow the README.md on contributing. Cheers!