Unused variable incorrectly reported by `jsonnet-lint`

Looks like the circumstance is very specific -- when the var is being used in an assert statement as part of an object.

**Expectation**: jsonnet-lint does not report any error in this source file.
**Actual**: jsonnet-lint reports unused variable error
**Details**:
```
% cat assert_lint.jsonnet
local myvar = 'xxxxx';

{
assert std.length(myvar) == 5 : 'yyyyy',
some_field: 'zzzzz',
}

% jsonnet-lint --version
Jsonnet linter v0.20.0

% jsonnet-lint assert_lint.jsonnet
assert_lint.jsonnet:1:7-22 Unused variable: myvar

local myvar = 'xxxxx';

Problems found!
```