[dotnet] Complete rename of DOTNET to SYSTEM_DOTNET. (#14535)

This was supposed to happen in d3643414e7, but
it looks like due to a merge conflict, this remaining part disappeared.

Make.config

@@ -529,12 +529,12 @@ $(TOP)/dotnet.config: $(TOP)/eng/Versions.props
 	$(Q) mv $@.tmp $@
 
 ifneq ("$(wildcard /usr/local/share/dotnet/dotnet)","")
-DOTNET=/usr/local/share/dotnet/dotnet
+SYSTEM_DOTNET=/usr/local/share/dotnet/dotnet
 else
 ifneq ("$(wildcard /usr/local/share/dotnet/x64/dotnet)","")
-DOTNET=/usr/local/share/dotnet/x64/dotnet
+SYSTEM_DOTNET=/usr/local/share/dotnet/x64/dotnet
 else
-DOTNET=/must/install/dotnet
+SYSTEM_DOTNET=/must/install/dotnet
 endif
 endif