Switch back to online installer for smoke tests

[mhegazy]

This reverts my previous change in #1032 to disable online installer for smoke tests.

Current explanation is that the test machine is still doing some setup when we start the job. With offline-installer we download the whole thing before starting setup, with online-installer we start setup much sooner; this results in a higher probability of another setup process interfering with the Swift toolchain install.

This PR changes the order of operations so we checkout code before we install the Swift SDK. in my tests this seems to do the trick.

here are a few runs that i kicked off with this change:

• https://github.com/thebrowsercompany/swift-build/actions/runs/18018427910
• https://github.com/thebrowsercompany/swift-build/actions/runs/18018431841
• https://github.com/thebrowsercompany/swift-build/actions/runs/18018436405
• https://github.com/thebrowsercompany/swift-build/actions/runs/18018441143
• https://github.com/thebrowsercompany/swift-build/actions/runs/18018447107