patch: cmd/go: go test -cachelink

[bradfitz]

I'd like to cherry-pick this into our Go tree, for future use in our CI, in https://github.com/tailscale/gotst/:

https://go-review.googlesource.com/c/go/+/739161

cmd/go: add 'go test -cachelink' boolean flag

This permits (opt-in via a new flag) running package tests faster in
subsequent runs with different -test.run=... flags, at the expense of
writing more to the build cache.

From the change:

	-cachelink
	    Cache the emphemeral test binary (the linker output) in the build
	    cache. This avoids re-linking the test binary if running the same
	    test binary repeatedly but with different flags, environment,
	    or other test inputs such that the test caching rules above
	    would not apply. On such case is Go test wrappers that shard tests
	    within a package across multiple machines, sharing a common build
	    cache with GOCACHEPROG.

/cc @tailscale/tooling

[bradfitz]

Go 1.25: 779d878
Go 1.26: 64a6cb4

/cc @tailscale/tooling