os/signal: signal.NotifyContext uses error cause that doesn't match context.Canceled [1.26 backport]

[gopherbot]

@ianlancetaylor requested issue #77639 to be considered for backport to the next 1.26 minor release.

@gopherbot Please open a backport to 1.26.

CL 721700 change the behavior of context.Cause on a context returned by signal.NotifyContext: it no longer matches context.Canceled. It's not clear to me that this is, strictly speaking, a breaking change, but we can at least adjust os/signal so that existing code continues to work as expected.

[gopherbot]

Change https://go.dev/cl/784100 mentions this issue: [release-branch.go1.26] os/signal: make NotifyContext Cause match context.Canceled