Restore `ENABLE_SMOKETESTS` for running smoketests (#3799)

## Motivation and Context
This PR restores [an environment
variable](https://github.com/smithy-lang/smithy-rs/pull/3758/files/62ff67ed89b1d920540b03a252a69bf50a1d1081#diff-903f493806a39cd49c26728eee0410545eb911162b1d889dd7154eae81b7c7c7)
we attempted to add in
https://github.com/smithy-lang/smithy-rs/pull/3758.

## Description
We removed that environment variable since we ran into an error
`ENABLE_SMOKETESTS: unbound variable` in our release pipeline. This PR
will have a default value `false` when `ENABLE_SMOKETESTS` is not
present.

We expect the value to be `false` in smithy-rs CI and `true` in our
release pipeline (can set it to `true` in our release pipeline
asynchronously).

## Testing
- Existing tests in CI

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