lang/swift6: Unbreak stage phase with unprivileged users

Stripping should have been done before COPYTREE_BIN, which drops the
writable bit from the executables and leads to "permission denied"
error.

PR: 296382
Reported by: kib
Tested by: kib
Fixes: 18ac8a4bf341 (lang/swift6: Add port: Powerful, flexible, multiplatform programming language (Version 6))