Skip native registrar builds in mmp/mtouch on Linux (require Xcode)

Co-authored-by: rolfbjarne <249268+rolfbjarne@users.noreply.github.com>

Author: Copilot

tools/mmp/Makefile

@@ -74,11 +74,13 @@ Microsoft.macOS.registrar.coreclr.arm64.m: $(TOP)/src/build/dotnet/macos/64/Micr
 %.arm64.a: %.arm64.m
 	$(Q_CC) $(CLANG) -isysroot $(macos_SDK) -DDEBUG -g -gdwarf-2 -x objective-c++ -std=c++14 -o $@ -c -arch arm64 $< -Wall -Wno-unguarded-availability-new -I$(TOP)/runtime -mmacosx-version-min=$(DOTNET_MIN_MACOS_SDK_VERSION) -fobjc-runtime=macosx $(EXTRA_DEFINES)
 
+ifndef IS_LINUX
 dotnet: $(MMP_TARGETS_DOTNET)
 ifdef INCLUDE_MAC
 install-local:: $(MMP_TARGETS_DOTNET)
 all-local:: $(MMP_TARGETS_DOTNET)
 endif
+endif
 
 clean-local::
 	rm -rf bin obj

tools/mtouch/Makefile

@@ -85,9 +85,11 @@ endef
 
 $(foreach platform,$(DOTNET_PLATFORMS_MTOUCH),$(foreach rid,$(DOTNET_$(platform)_RUNTIME_IDENTIFIERS),$(eval $(call InstallRegistrar,$(platform),$(rid)))))
 
+ifndef IS_LINUX
 dotnet: $(TARGETS_DOTNET)
 install-local:: $(TARGETS_DOTNET)
 all-local:: $(TARGETS_DOTNET)
+endif
 
 clean-local::
 	rm -Rf bin obj