Add additional locations for auto-discovery to scan

[Okeanos]

Add additional locations for auto-discovery to scan

Resolves #129

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Since 3f9736c running tests locally (on macOS) fails the build; I have not yet had the chance to debug this fully and understand whether it's a setup problem on my machine.

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[laeubi]

@gnodet can you take a look?

[gnodet]

Which tool uses this directory ?

[Okeanos]

This is based on the proposed alignment with the Gradle toolchain detection I mentioned in #129 – the original proposal for Gradle is documented in gradle/gradle#23620 and boils down to "people put it there during manual installations as common default".

I have no strong opinion on this and I am willing to exclude this particular search folder as it is a "customary" but ultimately a user-defined location.

[Okeanos]

@gnodet is there anything from my side still missing for this to proceed? If so, please let me know and I'll have a look. Other than that I'd be interested to know when this might be merged and potentially a new release of the Toolchains Plugin be released.