format string error

`tryFetchFromSubmodules` has a format string error:

```
for i := range repos {
subpath := paths[i]
subr := repos[i]
c.infof(" path=%q subr=%q\n", subpath, subr)
```

Outputs:

```
Searching submodules for: decbde0a79d6462b8a9c009fddc24d5c40dd913b/oss@25b021388b 25b021388b83c80d1c717479ba9694e713af989d
path="oss" subr=&{%!q(*filesystem.Storage=&{0xc000adf960 0xc000afea00 {{false false 0} 0xc000d84960 0xc000afea00 map[] [] 0 map[]} {0xc000afea00} {0xc000afea00} {0xc000afea00} {0xc000afea00} {0xc000afea00}}) map[] %!q(*chroot.ChrootHelper=&{0xc00000f740 /home/bradfitz/src/tailscale.io/oss})}
...found! in oss
```