Add *.tgz to .gitignore (radius-project#8020)

# Description

Building a private Bicep extension for local testing will create a
`.tgz` file. We shouldn't check these in.

## Type of change

- This pull request is a minor refactor, code cleanup, test improvement,
or other maintenance task and doesn't change the functionality of Radius
(issue link optional).



## Contributor checklist
Please verify that the PR meets the following requirements, where
applicable:

- [ ] An overview of proposed schema changes is included in a linked
GitHub issue.
- [ ] A design document PR is created in the [design-notes
repository](https://github.com/radius-project/design-notes/), if new
APIs are being introduced.
- [ ] If applicable, design document has been reviewed and approved by
Radius maintainers/approvers.
- [ ] A PR for the [samples
repository](https://github.com/radius-project/samples) is created, if
existing samples are affected by the changes in this PR.
- [ ] A PR for the [documentation
repository](https://github.com/radius-project/docs) is created, if the
changes in this PR affect the documentation or any user facing updates
are made.
- [ ] A PR for the [recipes
repository](https://github.com/radius-project/recipes) is created, if
existing recipes are affected by the changes in this PR.

Signed-off-by: Ryan Nowak <[email protected]>

.gitignore

@@ -66,3 +66,6 @@ tmp-*
 
 # Auto-generated Bicep markdown files
 hack/bicep-types-radius/generated/**/*.md
+
+# Bicep extensions
+*.tgz