Pass through environment variables when `SetEnv()` is used

## Background

Currently when `SetEnv()` is used, for example to set a custom plugin cache directory

```go
tf.SetEnv(map[string]string{
"TF_PLUGIN_CACHE_DIR": "./foobar",
})
```
then Terraform receives any environment variables set in the map + a handful of default Terraform (`TF_*`) ones we set internally.

Without `SetEnv()` we transparently pass through [almost] all environment variables, except for the Terraform ones which we want to be set explicitly as setting these would affect the behaviour/implementation which relies on specific output format or behaviour (e.g. no log lines in the output and no colouring).

This has a potentially unexpected negative side effect in that variables such as `TMP`, `PATH` and other common ones, which the Go stdlib relies on, are unavailable, breaking certain parts of the Terraform workflow.

For example, it is not possible to run `terraform init` on Windows without `TMP` unless it runs with elevated permissions, because Terraform uses `ioutil.TempFile()`. Temporary files are created in `$TMP` on Windows, which is typically set to `C:\Users\Radek\AppData\Local\Temp` which only needs permissions of a regular user. If that variable isn't available, then Go defaults to `C:\Windows` which typically requires elevated permissions.

## Proposal

Consider passing through all environment variables when `SetEnv()` is used, or allow-list a handful of common ones, such as `TMP`.

## Workaround

It is possible to work around this problem using e.g.

```go
tf.SetEnv(map[string]string{
"TF_PLUGIN_CACHE_DIR": "./foobar",
"TMP": os.Getenv("TMP"),
})
```
but it's likely there are other parts of Terraform which rely on other variables. e.g. I can imagine the need for `HOME` (Unix) `HOMEPATH` (Windows) or `APPDATA` in order to look up the CLI config file. https://developer.hashicorp.com/terraform/cli/config/config-file