Update swift-driver checkout

[etcwilde]

update-checkout should be checking out the release/6.0 driver for Swift-6.0 builds instead of the main branch driver.

[etcwilde]

@swift-ci please test

[etcwilde]

Windows failed to update llvm-project.

======UPDATE FAILURES======
C:\Users\swift-ci\jenkins\workspace\swift-PR-windows\llvm-project failed (ret=128): ['git', 'fetch', '--recurse-submodules=yes', '--tags']
b'fatal: protocol error: bad pack header\n'
update-checkout failed, fix errors and try again

Unrelated to this PR.

[etcwilde]

@swift-ci please test Windows platform