[lldb][test] Fix TestUseSourceCache for readonly source trees (llvm#113251)

TestUseSourceCache attempts to write to a build artifact copied from the
source tree, and asserts the write succeeded. If the source tree is read
only, the copy will also be read only, causing it to fail. When
producing the build artifact, ensure that it is writable.

Author: rupprecht

lldb/test/API/commands/settings/use_source_cache/Makefile

@@ -6,3 +6,4 @@ include Makefile.rules
 # Copy file into the build folder to enable the test to modify it.
 main-copy.cpp: main.cpp
 	cp -f $< $@
+	chmod u+w $@