docs: Update CONTRIBUTING.md for the provider repo

[programmer-ke]

While updating Akash CLI installation from source instructions in the website documentation (via akash-network/website#773), I noticed that some steps in the installation process in the CONTRIBUTING.md are likely not needed:

• direnv allow already creates the .cache directory so make cache does nothing
• make setup rule does not exist
• .env file is already tracked in the repo and .env.example doesn't exist