Allow to fail to delete rpath when not found

the install script has been broken since swiftlang/swift-package-manager@8876f87
which stops adding toolchain rpath for older OSes without system
swift libraries.

Author: kateinoigakukun

Utilities/build-script-helper.py

@@ -87,7 +87,8 @@ def install_binary(exe, source_dir, install_dir, toolchain):
 def delete_rpath(rpath, binary):
   cmd = ["install_name_tool", "-delete_rpath", rpath, binary]
   print(' '.join(cmd))
-  subprocess.check_call(cmd)
+  # allow failing when the given rpath is not found
+  subprocess.call(cmd)
 
 
 def handle_invocation(swift_exec, args):