[go] update toolchain go 1.26.4 -> 1.26.5

https://groups.google.com/g/golang-announce/c/OrmQE_Yp5Sc

Vulnerability #1: GO-2026-5856
    Invoking Encrypted Client Hello privacy leak in crypto/tls
  More info: https://pkg.go.dev/vuln/GO-2026-5856
  Standard library
    Found in: crypto/tls@go1.26.4
    Fixed in: crypto/tls@go1.26.5
    Example traces found:
      #1: reapi/proxy.go:86:24: reapi.Serve calls grpc.Server.Serve, which eventually calls tls.Conn.Handshake
      #2: build/statusz.go:73:15: build.NewStatuszServer calls http.Server.Serve, which eventually calls tls.Conn.HandshakeContext
      #3: hashfs/fs.go:854:22: hashfs.HashFS.ReadFile calls io.ReadFull, which eventually calls tls.Conn.Read
      #4: o11y/trace/trace.go:593:20: trace.Tracer.Close calls bufio.Writer.Flush, which calls tls.Conn.Write
      #5: subcmd/version/version.go:262:36: version.Command.sisoCommit calls http.Client.Do, which eventually calls tls.Dialer.DialContext

Change-Id: I2ceab1efec11ee980ac8d3e2b9506d00baa8583d
Reviewed-on: https://chromium-review.googlesource.com/c/build/+/8082661
Auto-Submit: Fumitoshi Ukai <ukai@google.com>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Richard Wang <richardwa@google.com>
Reviewed-by: Philipp Wollermann <philwo@google.com>