[Improvement] Add missing `readonly` when applicable

[Evangelink]

There are a couple of locations in the codebase where some declared fields could be marked as readonly because they are only initialized in the constructor.

Detailed Description

List of files:

• GlobbingResolver.cs
• OverrideConfigOptionParser.cs
• ProgramFixture.cs
• GitPreparer.cs
• RetryAction.cs
• VersionInBranchNameVersionStrategy.cs
• GitRepository.cs
• GitRepositoryInfo.cs