Drop custom symlink for generic plugin cache setting

[skorfmann]

Since the current symlink approach for caching poviders has multiple
drawbacks, this PR switches to the general plugin cache dir supported
by Terraform directly.

https://www.terraform.io/docs/commands/cli-config.html#provider-plugin-cache

Fixes #189
Fixes #222
Fixes #349

[skorfmann]

Looks like lots of parallel provider downloads and TF_PLUGIN_CACHE_DIR aren't working well together - I know I had similar issues with Terragrunt in another project.

Perhaps we could think about adding all used providers to the Docker image in advance - Or have one bootstrap step where all used providers are fetched sequentially.

[skorfmann]

Again, it'd be pretty helpful to have the Docker image published automatically (#36) /cc @anubhavmishra

[anubhavmishra]

We can add a note in the docs to show that we are setting the TF_PLUGIN_CACHE_DIR environment variable.

[skorfmann]

We can add a note in the docs to show that we are setting the TF_PLUGIN_CACHE_DIR environment variable.

Done

[github-actions]

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