Use Swift 6.0 for tests

[lann]

Currently just testing CI here...

Swift 5.x uses an old wasi-sdk which might make it susceptible to WebAssembly/wasi-libc#377 when used with WAGI.

[rylev]

Looks like the Swift installation wasn't quite right:

stdout:
Building component test with `swiftc -target wasm32-unknown-wasi main.swift -o main.wasm`

stderr:
error: missing external dependency '/opt/hostedtoolcache/swiftwasm/wasm-6.0-SNAPSHOT-2024-07-20-a/x86_64/usr/lib/swift/wasi/static-executable-args.lnk'

[lann]

Yeah I had the same issue locally; I was hoping a "more standard" environment would fix it.