x/build: add darwin builders with macOS 26 (macOS Tahoe)

[dmitshur]

This is a tracking issue for adding darwin/arm64 and darwin/amd64 builders with macOS 26 (macOS Tahoe).

CC @golang/release, @golang/darwin.

[gabyhelp]

Related Issues

• x/build: add darwin builders with macOS 14 (macOS Sonoma) #60610 (closed)
• x/build: have darwin/arm64 builders on macOS 13 (Ventura) #55354 (closed)
• x/build: have darwin/amd64 builders on macOS 13 (Ventura) #55355 (closed)
• x/build: have more than 1 macOS 12.0 Monterey builder (arm64/amd64) #49149 (closed)
• x/build: add a darwin/amd64 longtest builder #35678 (closed)
• x/build: add darwin-10_14 builder #27806 (closed)
• x/build: add macOS Catalina builder (darwin/amd64 only) #34748 (closed)
• x/build: add darwin-race builder #17674 (closed)
• x/build/cmd/release: use darwin-amd64-11_0 releaselet starting with Go 1.17 Beta 1 #46161 (closed)
• x/build: add macOS 11.0 Big Sur builder (darwin/amd64) #40762 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}