CI: Use Swift SDK by default

[kateinoigakukun]

In other words, drop toolchain-style installation support.

swiftwasm/swift#5604

[github-actions]

Time Change: +234ms (2%)

Total Time: 9,711ms

Test name	Duration	Change
Serialization/JavaScript function call through Wasm import	26ms	+7ms (25%)	🚨
Serialization/JavaScript function call through Wasm import with int	15ms	+1ms (8%)	🔍
Serialization/Swift Int to JavaScript with call	1,069ms	+59ms (5%)	🔍

View Unchanged

Test name	Duration	Change
Serialization/JavaScript function call from Swift	112ms	-3ms (2%)
Serialization/Swift Int to JavaScript with assignment	352ms	+9ms (2%)
Serialization/JavaScript Number to Swift Int	297ms	+10ms (3%)
Serialization/Swift String to JavaScript with assignment	413ms	+17ms (3%)
Serialization/Swift String to JavaScript with call	1,123ms	+49ms (4%)
Serialization/JavaScript String to Swift String	3,631ms	+63ms (1%)
Object heap/Increment and decrement RC	2,659ms	+20ms (0%)

View Baselines

Test name	Duration
Serialization/Call JavaScript function directly	3ms
Serialization/Assign JavaScript number directly	4ms
Serialization/Call with JavaScript number directly	3ms
Serialization/Write JavaScript string directly	2ms
Serialization/Call with JavaScript string directly	3ms