Fix maccatalyst RID detection in SDK targets

The OS detection logic falls through to 'linux' for maccatalyst-* RIDs
since they don't start with 'win' or 'osx'. Add explicit handling for
the 'maccatalyst' prefix to map it to 'osx', so the correct Darwin CLI
binary is downloaded.

Relates to #454

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

dotnet/src/build/GitHub.Copilot.SDK.targets

@@ -8,6 +8,7 @@
     <!-- Determine OS: from RID prefix if set, otherwise from build host -->
     <_CopilotOs Condition="'$(RuntimeIdentifier)' != '' And $(RuntimeIdentifier.StartsWith('win'))">win</_CopilotOs>
     <_CopilotOs Condition="'$(_CopilotOs)' == '' And '$(RuntimeIdentifier)' != '' And $(RuntimeIdentifier.StartsWith('osx'))">osx</_CopilotOs>
+    <_CopilotOs Condition="'$(_CopilotOs)' == '' And '$(RuntimeIdentifier)' != '' And $(RuntimeIdentifier.StartsWith('maccatalyst'))">osx</_CopilotOs>
     <_CopilotOs Condition="'$(_CopilotOs)' == '' And '$(RuntimeIdentifier)' != ''">linux</_CopilotOs>
 
     <!-- Determine arch: from RID suffix if set, otherwise from build host -->