Package plugins don't work on Windows if the PATH environment variable isn't accessed with the right case

[jakepetroules]

Builds of packages using package plugins may fail on Windows with the following error:

error: plugin process ended by an uncaught signal: 309 <command: C:\foo\.build\plugins\cache\myplugin.exe>, <output:
>

309 translates to 0x135, which maps to 0xc0000135 aka STATUS_DLL_NOT_FOUND.

This is due to the following code in DefaultPluginScriptRunner.swift:

#if os(Windows)
        let pluginLibraryPath = self.toolchain.swiftPMLibrariesLocation.pluginLibraryPath.pathString
        var env = ProcessInfo.processInfo.environment
        if let Path = env["Path"] {
            env["Path"] = "\(pluginLibraryPath);\(Path)"
        } else {
            env["Path"] = pluginLibraryPath
        }
        process.environment = env
#endif

Environment variable names are case-insensitive on Windows. On a real Windows host, it tends to be spelled "Path". In a Windows Container (Docker), it tends to be spelled "PATH".

The code will end up clearing out the other paths from the PATH environment variable in the else case because Path != PATH.

We need to access the path here in a case-insensitive manner -- use the existing Environment abstraction for this, which handles Windows case sensitivity concerns.

Closes #8125

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[jakepetroules]

@swift-ci please smoke test

[jakepetroules]

@swift-ci please smoke test

[jakepetroules]

@swift-ci test