[release-branch.go1.17] [tailscale1.17] go/build: update for //go:bui…

…ld lines

Look for //go:build ignore, not // +build ignore, in deps_test.go.

For golang#41184.

Change-Id: Iba8617230aa620223e2bc170f18d0c54557318c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/359315
Trust: Russ Cox <rsc@golang.org>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

(cherry picked from commit golang.org/cl/359315)

src/go/build/deps_test.go

@@ -615,7 +615,7 @@ func TestDependencies(t *testing.T) {
 	}
 }
 
-var buildIgnore = []byte("\n// +build ignore")
+var buildIgnore = []byte("\n//go:build ignore")
 
 func findImports(pkg string) ([]string, error) {
 	vpkg := pkg