x/build: gopherbot labelCompilerRuntimeIssues should use closest matching prefix

[prattmic]

#80397 is a compiler/runtime issue but did not get labeled by gopherbot.

The problem is the package name: cmd/compile/internal/noder.

This package is missing from https://dev.golang.org/owners, though cmd/compile is listed there.

gopherbot requires an exact match: https://cs.opensource.google/go/x/build/+/master:cmd/gopherbot/gopherbot.go;l=1129;drc=74861f19d89a83ca9474aad1aef251618dc62f06

Better would be for gopherbot to allow matching on a prefix if there isn't a more precise prefix with different owners (though I'm not sure that compiler/runtime actually has any of those).

[gopherbot]

Change https://go.dev/cl/802960 mentions this issue: cmd/gopherbot: use longest prefix matching for compiler/runtime issue labeling