Fix no-prototype-builtins issue in Ruleset variables()

[Krinkle]

What:

div {
  hasOwnProperty: blue;
}

http://lesscss.org/less-preview/#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

Why:

t.indexOf is not a function

The added test fails in CI without this patch as:

- /home/runner/work/less.js/less.js/packages/test-data/less/_main/property-name-interp: OK
ERROR: ruleList.indexOf is not a function

TypeError: ruleList.indexOf is not a function
    at ToCSSVisitor._removeDuplicateRules (/home/runner/work/less.js/less.js/packages/less/lib/less/visitors/to-css-visitor.js:282:34)
    at ToCSSVisitor.visitRuleset (/home/runner/work/less.js/less.js/packages/less/lib/less/visitors/to-css-visitor.js:232:18)
    at Visitor.visit (/home/runner/work/less.js/less.js/packages/less/lib/less/visitors/visitor.js:67:32)
    at Visitor.visitArray (/home/runner/work/less.js/less.js/packages/less/lib/less/visitors/visitor.js:105:30)
    at Ruleset.accept (/home/runner/work/less.js/less.js/packages/less/lib/less/tree/ruleset.js:42:34)
    at ToCSSVisitor.visitRuleset (/home/runner/work/less.js/less.js/packages/less/lib/less/visitors/to-css-visitor.js:227:25)
    at Visitor.visit (/home/runner/work/less.js/less.js/packages/less/lib/less/visitors/visitor.js:67:32)
    at ToCSSVisitor.run (/home/runner/work/less.js/less.js/packages/less/lib/less/visitors/to-css-visitor.js:81:30)
    at default_1 (/home/runner/work/less.js/less.js/packages/less/lib/less/transform-tree.js:78:21)
    at ParseTree.toCSS (/home/runner/work/less.js/less.js/packages/less/lib/less/parse-tree.js:18:58)
- /home/runner/work/less.js/less.js/packages/test-data/less/_main/scope: OK

I note that it doesn't say which LESS file the failure happened in, or which test case it was running (both would point to rulesets.less in this case).

Checklist:

• Documentation: N/A
• [x] Added/updated unit tests
• [x] Code complete