Ignore PROJECT variable for non .NET project files

[Saipriya-1144]

In .NET detection, PROJECT environment variable is used to specify which project file to build, but this environment variable is very generic and may not contain valid .NET project files.

Now only honor PROJECT when it specifies valid .csproj or .fsproj files, otherwise ignore and fall back to auto-detection.

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• [ ] Tests are included and/or updated for code changes.
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