os/mac/diagnostic: check min CLT version on env setup

[Bo98]

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This covers the case where the user has CLT 12.0-12.4 on Big Sur and is setting up the build env via non-build-from-source means (e.g. bottle :unneeded). We no longer support those versions of the CLT and they will throw errors if used.

[BrewTestBot]

Review period skipped due to critical label.

[boblail]

Tested this with a bottle :unneeded formula (and old CLT) — looks good 👍

[MikeMcQuaid]

Nice fix, thanks @Bo98!

[Bo98]

I might tag 3.2.8 in a bit with this fix as I suspect there will be a few bug reports. There isn't much other code changing commits since the last one too so should be low risk.

[Bo98]

3.2.8 is released now. Thanks @boblail for the initial report and testing this out.