Use package traits for disabling jemalloc

[parkera]

It would be nice to use the new Swift 6.1 feature of package traits to disable the jemalloc integration, rather than relying on an environment variable here.

[hassila]

@parkera definitely agree, that would be nice - but still supporting older toolchains (expect to drop 5.x when 6.2 rolls out)

[parkera]

You should be able to use the version-specific manifest to support this by creating a 5.x Package.swift and then setting swift-tools-version to 6.1. See here for more details.

[hassila]

See also https://github.com/ordo-one/package-benchmark/issues/318

[parkera]

Thanks!