Fixes #32028: Investigate Template Baseline tests errors

[DavidKarlas]

Latest dotnet new doesn't work with relative path for --debug:custom-hive... Hence make it absolute
Also switched to using --debug:disable-sdk-templates to avoid potential conflict between SDK and tests installed templates.

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PR Title
Fixes #32028: Investigate Template Baseline tests errors

PR Description
In dotnet new we recently changed how SDK(and optional workloads) templates are installed... Which made it impossible to uninstall them... Hence we introduced --debug:disable-sdk-templates to make it possible to test SDK templates in this repo...
But it looks like we introduced another regression which is, --debug:custom-hive doesn't support relative path anymore... I already open issue on our repo to fix that, but since propagation of that would take some time, I made change here to use Path.GetFullPath()

Addresses #32028

[HaoK]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[dougbu]

Many thanks @DavidKarlas.

We no longer need this PR because @halter73 included the fix in 'release/6.0-preview4' as part of 90137b0 / #32003. That merged into main in 12401e7 / #32123.