Add Swift 5.6 compilers (error w/ concurrency things)

[helje5]

So I setup a 5.6 build script and formulas for Ubuntu 20.04, but they don't fly yet due to missing concurrency stuff.

When trying to compile a project, I get:

$ swift build --destination \
>   ${HOMEBREW_PREFIX}/lib/swift/dst/x86_64-unknown-linux/swift-5.6-ubuntu20.04.xtoolchain/destination.json

Building for debugging...
warning: Could not read SDKSettings.json for SDK at: /opt/homebrew/lib/swift/dst/x86_64-unknown-linux/swift-5.6-ubuntu20<unknown>:0: error: could not find module '_Concurrency' for target 'x86_64-unknown-linux'; found: x86_64-unknown-linux-gnu, at: /opt/homebrew/Cellar/spm-dest-5.6-x86_64-ubuntu20.04/5.6.2_20/lib/swift/dst/x86_64-unknown-linux/swift-5.6-ubuntu20.04.xtoolchain/swift.xctoolchain/usr/lib/swift/linux/_Concurrency.swiftmodule
<unknown>:0: error: could not find module '_Concurrency' for target 'x86_64-unknown-linux'; found: x86_64-unknown-linux-gnu, at: /opt/homebrew/Cellar/spm-dest-5.6-x86_64-ubuntu20.04/5.6.2_20/lib/swift/dst/x86_64-unknown-linux/swift-5.6-ubuntu20.04.xtoolchain/swift.xctoolchain/usr/lib/swift/linux/_Concurrency.swiftmodule

[octo47]

I found it can be fixed with changing target in
/opt/homebrew/lib/swift/dst/x86_64-unknown-linux/swift-5.6-ubuntu20.04.xtoolchain/destination.json

If you replace "target": "x86_64-unknown-linux" with "target":"x86_64-unknown-linux-gnu" it will work

I have not much experience with this tap, so can't say where it need to be fixed