getPackageSuppliers already handles SkipPackageSuppliers.none correctly,
no need for an extra check.

For some reason, dflags and lflags were not using the 'add' convenience function,
which allows to deduplicate flags.
As a result some flags, especially link flags which propagate upwards,
ended up taking half of the command line length.

PR #2016 introducted a de-duplication strategy that is too eager and corrupts argument chains, such as "-framework CoreServices -framework CoreFoundation" where the "-framework" argument needs to be duplicated for each framework.

This reverts commit 10bea6f.

Due to a security vulnerability, Github hass removed set-env and add-path.
See https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/