Fix exception import Az.Accounts caused by env var

[isra-fel]

Description

Fix #18304

As stated in the issue, on non-Windows OS, it is possible to have multiple environment variables which only differ by case.
The original code didn't take this into consideration when listing all the environment variables and putting them in a case-insensitive dictionary using IEnumerable.ToDictionary() which throws an exception in the above scenario because of duplicated dictionary keys.

The solution is to write the logic of converting IEnumerable to Dictionary manually and overwrite previous value when keys are the same.

I'm thinking, to make config framework more robust against unexpected errors, we can do something like this:

// in AzureSessionInitializer.cs
try
{
    configInitializer.Initiaize();
}
catch (e)
{
    WriteWarning(e);
    configInitializer.InitilzeInSafeMode(); // initialize config framework without config providers (json, env var, in-memory). Only default values are available.
}

I will cover this in another PR.

Checklist

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• The PR does not introduce breaking changes
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  • a cmdlet design review was approved for the changes in this repository (Microsoft internal only)
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[daandupau]

Can this be merged, our pipelines are not working because of this.

[BasBerk]

Here the same, this bug is causing us major headaches

[StoneNLD]

pipelines are not working with task AzurePowershell anymore, please get this merged.