chore: modernize EnvironmentVariableService

[JamieMagee]

What

Cleans up EnvironmentVariableService.GetEnvironmentVariable:

• Replaces string.Compare(x, name, true) == 0 (a .NET Framework idiom) with string.Equals(x, name, StringComparison.OrdinalIgnoreCase).
• Short-circuits to Environment.GetEnvironmentVariable(name) on Windows, where the lookup is already case-insensitive — no need to enumerate every environment variable.
• Drops #nullable disable from the file.

Why

The original implementation predates net8.0 and always paid the cost of enumerating every environment variable, even on Windows where the platform handles case-insensitivity natively.

Part 4 of a 6-PR cleanup series removing .NET-Framework-era anachronisms.

[Copilot]

Pull request overview

Modernizes EnvironmentVariableService.GetEnvironmentVariable to better align with net8.0 and platform behavior, reducing unnecessary work on Windows while keeping cross-platform case-insensitive lookup behavior.

Changes:

• Updated IEnvironmentVariableService.GetEnvironmentVariable to return string? to reflect the fact it can return null.
• Optimized Windows behavior by short-circuiting to Environment.GetEnvironmentVariable(name) (no enumeration).
• Replaced string.Compare(..., true) with string.Equals(..., StringComparison.OrdinalIgnoreCase) and removed #nullable disable from the implementation file.

Show a summary per file

File	Description
src/Microsoft.ComponentDetection.Contracts/IEnvironmentVariableService.cs	Updates interface return nullability for GetEnvironmentVariable.
src/Microsoft.ComponentDetection.Common/EnvironmentVariableService.cs	Optimizes Windows lookup and modernizes case-insensitive comparison; enables nullable context.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 2

[github-actions]

👋 Hi! It looks like you modified some files in the Detectors folder.
You may need to bump the detector versions if any of the following scenarios apply:

• The detector detects more or fewer components than before
• The detector generates different parent/child graph relationships than before
• The detector generates different devDependencies values than before

If none of the above scenarios apply, feel free to ignore this comment 🙂