Add a new workflow to GitHub Actions for creating the test build of Custom GitHub actions

[eason9487]

Changes proposed in this Pull Request:

In the past, it was often necessary to use the Testing release way to create a prerelease to test a custom action. However, it's not so convenient for testing and requires creating and deleting prerelease and versioning tag.

To simplify the test, this PR adds a new workflow to GitHub Actions of this repo for creating the test build of Custom GitHub actions.

Screenshots:

Detailed test instructions:

💡 Since it's unable to actually test this PR before merging it into trunk, I verified its results in my forked repo and the eason9487@bebee50 commit.

1. View the workflow run to observe the creation of a test build based on trunk: https://github.com/eason9487/grow/actions/runs/6979381452
2. View the workflow run to observe the processing of testing the release process with a prerelease: https://github.com/eason9487/grow/actions/runs/6979399768
3. Read the Create a test build section in README to determine if it's clear and easy to understand.
4. Read the updated Testing the release process section to determine if it can tell a different use than the Create a test build section.

[tomalec]

I checked the code, and inspected the build runs on the fork repo. LGTM. Thanks :) I'm excited to use the new flow :)