x/build/dashboard: update darwin-arm64-11_0-toothrot builder macOS version to a newer point release

[dmitshur]

The new "Test execution environment" information in a recent build log (example) shows that darwin-arm64-11_0-toothrot is testing Go with darwin version xnu-7195.41.8~9, which corresponds to macOS Big Sur 11.0 (e.g., using this release history as one source).

https://support.apple.com/en-us/HT201222 describes this version as "Preinstalled only on certain Mac models; please update to macOS Big Sur 11.0.1". It might not be possible to update to 11.0.1 today since newer point releases have superseded it, but updating to a newer version such as 11.6.5 should be available and fine.

Doing so should help make the information the builder produces more representative of uses of Go on Big Sur. It may also make a difference on occasional "unexpected stale targets" failures (issue #49692, also recent example), though it's hard to predict.

CC @golang/release. (@toothrot asked to be assigned, so done, but please feel free to prioritize this maintenance opportunity vs others as needed.)

[toothrot]

Updated the machine.

Recent build log:

# OS Version: Darwin 20.6.0 Darwin Kernel Version 20.6.0: Tue Feb 22 21:10:42 PST 2022; root:xnu-7195.141.26~1/RELEASE_ARM64_T8101 arm64

This corresponds to 11.6.5 according to Wikipedia.

I've also unstacked the machines, which might have thermal benefits @bcmills, though they never feel particularly warm.