Undo breaking change in 6.0 in environment variable configuration prefix support

[maryamariyan]

When using environment variable configuration, brings back support for prefix being normalized like it was in dotnet 5.0.

The prefix is expected to use ":" to work (refer to updated tests).

Fixing regression from last release. Revert changes made in https://github.com/dotnet/runtime/pull/42932/files while updating tests to reduce ambiguity.

Fixes#61577

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Issue Details

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When using environment variable configuration, brings back support for prefix being normalized like it was in dotnet 5.0.

The environment variable prefix should contain : if on Windows, and __ if on Linux. But the prefix is expected to use ":" to work (refer to updated tests).

Fixing regression from last release. Revert changes made in https://github.com/dotnet/runtime/pull/42932/files while updating tests to reduce ambiguity.

Fixes #61577

Author:	maryamariyan
Assignees:	maryamariyan
Labels:	area-Extensions-Configuration
Milestone:	-

[maryamariyan]

/backport to release/6.0

[github-actions]

Started backporting to release/6.0: https://github.com/dotnet/runtime/actions/runs/1588873597