[build-script-helper] Prefer just-built plugins to SDK plugins

Add '-Xswiftc -plugin-path -Xswiftc ${toolchain}/lib/swift/host/plugins'
to swiftpm invocations.

Author: rintaro

Utilities/build-script-helper.py

@@ -84,6 +84,9 @@ def get_swiftpm_options(args):
       # Relative library rpath for swift; will only be used when /usr/lib/swift
       # is not available.
       '-Xlinker', '-rpath', '-Xlinker', '@executable_path/../lib/swift/macosx',
+      # Prefer just-built plugins to SDK plugins.
+      '-Xswiftc', '-plugin-path',
+      '-Xswiftc', os.path.join(args.toolchain, 'lib', 'swift', 'host', 'plugins')
     ]
   else:
     swiftpm_args += [