use statements can sometimes erroneously capture local variables

[mitchellwrosen]

I've seen this a few times over the last few months during upgrades and merges (has anyone else?), and I finally narrowed it down:

```ucm
scratch/main> builtins.merge lib.builtin
```

```unison
foo : Nat
foo =
  baz = bar.baz + bar.baz
  19

bar.baz : Nat
bar.baz = 20

qux : Nat
qux = foo + foo
```

```ucm
scratch/main> add
```

```unison
foo : Nat
foo =
  baz = bar.baz + bar.baz
  20

bar.baz : Nat
bar.baz = 20
```

```ucm:error
scratch/main> update
```

This update will fail, because it incorrectly renders foo as

foo : Nat
foo =
  use Nat +
  use bar baz
  baz = baz + baz
  20

rather than the correct

foo : Nat
foo =
  use Nat +
  baz = bar.baz + bar.baz
  20