fix: LineFilterLabelFilter.String() regexp correct delimiters (backport k277)

[loki-gh-app]

Backport 0303aee from #17129

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What this PR does / why we need it:
When the querysharder executes, it converts a query into an AST representation before forwarding it on to the queriers. Unfortunately, when this AST representation is converted back into a string, it does not handle the case where a regexp can contain backticks - it always tries to wrap the regexp in backticks, which can lead to parser errors. This PR updates the String() method that converted the filter back into a query string and adapts the delimiters based on the content of the regexp.

Which issue(s) this PR fixes:
Fixes #16674

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
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