Fix hang using multi-node package plugin on Swift 6+

[simonjbeaumont]

Motivation:

The integration tests for Swift 6.0 and 6.1 are hanging. They are passing on Swift 5.10. This is because SwiftPM started taking a lock over the package and cannot be run concurrently on the same package.

This is causing hangs in the integration test because it calls the multi-node package plugin which is running the MultiNodeTestKitRunner using swift run MultiNodeTestKitRunner.

This isn't the supported way for Swift package plugins to use executables from the package, which should instead be looked up from the context passed into the plugin.

Modifications:

Use the package plugin context to find the executable dependency used by the multi-node plugin.

Result:

Integration tests no longer hang on Swift 6.0 and 6.1.

[simonjbeaumont]

@czechboy0 @rnro this is a false positive from the API breakage checker, right? Can't see how it could be legit.