use defer to clean up mutex lock in loki/source/file/tailer.go

[edv1n]

PR Description

A small refactor of a previous PR to follow Uber Go Style Guide by using defer to clean up mutex.

This enhance readability and prevent forget to unlock the mutex in the future.

Which issue(s) this PR fixes

N/A

Notes to the Reviewer

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PR Checklist

N/A

[edv1n]

I am sorry, my bad. I thought I ran the test against my changes locally. Closing this PR as it doesn't work.

[dehaansa]

Appreciate the desire to improve Alloy! I wanted this to work, too, golang really could use a nicer feature for scoping mutexes.