Describe the bug
coverlet.msbuild.tasks overrides code coverage result file using file name of given CoverletOutput parameter.

To Reproduce

execute command:
dotnet test test\coverlet.core.tests\coverlet.core.tests.csproj --configuration debug /p:CollectCoverage=true /p:CoverletOutputFormat="json%2copencover" /p:CoverletOutput="C:\GitHub\coverlet\artifacts\Reports\coverlet.core\coverage.json" /p:Exclude="[coverlet.core.tests.samples.netstandard]*%2c[coverlet.tests.xunit.extensions]*%2c[coverlet.tests.projectsample]*%2c[testgen_]*" --results-directory:"C:\GitHub\coverlet\artifacts\Reports\coverlet.core" --diag:"C:\GitHub\coverlet\artifacts\log\debug\coverlet.core.test.diag.log;tracelevel=diag"

Expected behavior
2 code coverage result files are available (formats: json, opencover)

Actual behavior
Only one code coverage result file is available.

Configuration (please complete the following information):
Please provide more information on your .NET configuration:
* Which coverlet package and version was used? 6.0.1-preview.39.g9321d2e2cc
* Which version of .NET is the code running on? net8.0
* What OS and version, and what distro if applicable? windows 11
* What is the architecture (x64, x86, ARM, ARM64)? x64

Additional context
none