Big Sur fixes

[MikeMcQuaid]

• Make missing SDKs message more clear.
• Always refer to Big Sur as 11.0 (even on Intel).

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[MikeMcQuaid]

@Bo98 @claui you got a new CLT/Xcode version we can set as a new minimum here?

[Bo98]

Maybe change "CLT" to "Command Line Tools", if that's clearer?

We should also update the installer to (temporarily) not install the CLT on 11.0, regardless of the CPU, given softwareupdate is busted.

@Bo98 @claui you got a new CLT/Xcode version we can set as a new minimum here?

Xcode: I think all betas are currently tracked by Homebrew as "12.0".
CLT: version detection isn't consistent so difficult to bump this - sometimes we track the version based on Clang and sometimes based on the package version.

[MikeMcQuaid]

Seems OK as-is so let's merge and see if this improves things 🤞🏻

We should also update the installer to (temporarily) not install the CLT on 11.0, regardless of the CPU, given softwareupdate is busted.

Will open a PR for this.