Change channel passed to ci_setup to track runtime branch (#59071)

* Change channel passed to ci_setup to track runtime branch

With this change we move to specifing the channel that is passed to ci_setup,
instead of it always being main. The allows us to have the runtime branch
help decide the correct version of the sdk that we should pull down with
dotnet-install. This means that going forward, we will need to have channels
that match the names of all branches that we plan to run from the runtime
side. If we do not, the sdk installation step will fail during ci_setup.

* Change logic to not break private runs

The previous change here broke private runs as it required the channel map
to have an entry matching the name of the runtime branch you were testing.
This fix now defaults the channel to main for private runs.

Author: DrewScoggins

eng/testing/performance/performance-setup.ps1

@@ -97,7 +97,12 @@ if ($iOSMono) {
 }
 
 # FIX ME: This is a workaround until we get this from the actual pipeline
-$CommonSetupArguments="--channel main --queue $Queue --build-number $BuildNumber --build-configs $Configurations --architecture $Architecture"
+$CleanedBranchName = "main"
+if($Branch.Contains("refs/heads/release"))
+{
+    $CleanedBranchName = $Branch.replace('refs/heads/', '')
+}
+$CommonSetupArguments="--channel $CleanedBranchName --queue $Queue --build-number $BuildNumber --build-configs $Configurations --architecture $Architecture"
 $SetupArguments = "--repository https://github.com/$Repository --branch $Branch --get-perf-hash --commit-sha $CommitSha $CommonSetupArguments"
 
 if($NoPGO)