[ci] Fix reportgenerator installation error

[liushilongbuaa]

What I did
Fix pipeline error.
here is the error message:

/tmp/823d4a0e-75c7-4ca8-b487-06e1f38eae6d/restore.csproj : error NU1202: Package dotnet-reportgenerator-globaltool 5.4.1 is not compatible with net7.0 (.NETCoreApp,Version=v7.0) / any. Package dotnet-reportgenerator-globaltool 5.4.1 supports:
/tmp/823d4a0e-75c7-4ca8-b487-06e1f38eae6d/restore.csproj : error NU1202:   - net8.0 (.NETCoreApp,Version=v8.0) / any
/tmp/823d4a0e-75c7-4ca8-b487-06e1f38eae6d/restore.csproj : error NU1202:   - net9.0 (.NETCoreApp,Version=v9.0) / any

+ reportgenerator -reporttypes:Cobertura '-reports:tests/*coverage.xml' -targetdir:.
/agent/_work/_temp/057fa35b-9e5b-426a-bd88-2c5956cb8dcf.sh: line 2: reportgenerator: command not found

Why I did it

How I verified it
pipeline in this PR didn't shows that error message.
Details if related

[liushilongbuaa]

@prsunny , can you help merge this PR?
test case error is not caused by this PR.