(#74) Update prerelease tag if starts with digit

[corbob]

Description Of Changes

Update the prerelease tag to start with an a if it starts with a digit.

Motivation and Context

Chocolatey doesn't allow prerelease tags that start with a digit.

Testing

1. Ran ./build.bat
2. Copied generated nupkg contents into ChocolateyGUI project
3. Ran ./build.bat -Target Print-CI-Provider-Environment-Variables to verify that version number prints properly.

Change Types Made

• [x] Bug fix (non-breaking change)
• [ ] Feature / Enhancement (non-breaking change)
• [ ] Breaking change (fix or feature that could cause existing functionality to change)
• [ ] PowerShell code changes.

Related Issue

Fixes #74

Change Checklist

• [ ] Requires a change to the documentation
• [ ] Documentation has been updated
• [ ] Tests to cover my changes, have been added
• [ ] All new and existing tests passed.
• [ ] PowerShell v2 compatibility checked.

[gep13]

For now, the failing AppVeyor build is "expected", as we have removed AppVeyor from the build process. We need to get this updated to use GitHub Actions as the OSS build system so that we have consistency with the other OSS projects.

[gep13]

@corbob thank you for getting this added!