PR checks: stop setting experimental Swift var for new CLI versions

[angelapwen]

Now that latest and cached are both 2.13.3, which is the version in which we GA'ed Swift, we should stop setting this experimental variable when we test these CLI versions so we can test the case where the variable is unset.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[henrymercer]

Thanks! Separately, we might want to rename cached to something like ghes-default — on Dotcom with controlled switchover we will follow the feature flags rather than what's in the toolcache.