Add back -no-toolchain-stdlib-rpath when installing sourcekit-lsp

Pull #597 incorrectly removed this, as it's still needed on ELF platforms.

Author: finagolfin

Utilities/build-script-helper.py

@@ -187,6 +187,8 @@ def build_single_product(product: str, swift_exec: str, args: argparse.Namespace
     Build one product in the package
     """
     swiftpm_args = get_swiftpm_options(swift_exec, args)
+    if args.action == 'install':
+      swiftpm_args += ['-Xswiftc', '-no-toolchain-stdlib-rpath']
     additional_env = get_swiftpm_environment_variables(swift_exec, args)
     cmd = [swift_exec, 'build', '--product', product] + swiftpm_args
     check_call(cmd, additional_env=additional_env, verbose=args.verbose)