internal/vulncheck: refactor a loop with an append

This makes staticcheck happy and resolves builders' failures.

Change-Id: I90c0aa18ee7ca35fd18879d1d4dd5b6c9b0bcfa1
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/560335
Run-TryBot: Zvonimir Pavlinovic <zpavlinovic@google.com>
Reviewed-by: Maceo Thompson <maceothompson@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/vulncheck/binary.go

@@ -159,9 +159,7 @@ func allKnownVulnerableSymbols(affVulns affectingVulns) map[string][]string {
 						syms = []string{fmt.Sprintf("%s/*", p.Path)}
 					}
 
-					for _, symbol := range syms {
-						pkgSymbols[p.Path] = append(pkgSymbols[p.Path], symbol)
-					}
+					pkgSymbols[p.Path] = append(pkgSymbols[p.Path], syms...)
 				}
 			}
 		}