Fix download filename for RC versions

[aaylward]

fixes #32

What is this?

This PR fixes filename resolution for release candidate version of protoc. RC names like 29.0-rc3 have release artifacts with names like 29.0-rc-3.... see the release assets for 29.0-rc3 or 28.0-rc2

Changes are visible to end-users: no

Test plan

• Pretty much covered by existing test cases with example protoc version changed to LATEST
• Added unit test to validate release_version -> artifact_name against supported versions

[CLAassistant]

All committers have signed the CLA.

[alexeagle]

thanks!

[alexeagle]

Hm, I'm not able to push to your branch

To github.com:aaylward/toolchains_protoc.git
 ! [remote rejected] aaylward/main -> aaylward/main (permission denied)
error: failed to push some refs to 'github.com:aaylward/toolchains_protoc.git'

Will land in #37