go/analysis/singlechecker: respect custom FlagSet

[AkihiroSuda]

When a custom flag.FlagSet is provided, the args should be fetched from that custom FlagSet,
not from the global os.Args.

This fix is useful for calling singlechecker from a CLI subcommand like
gosocialcheck run ./...: AkihiroSuda/gosocialcheck#1

Fixes golang/go#73875

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Akihiro Suda:

Patch Set 1:

(1 comment)

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Please don’t reply on this GitHub thread. Visit golang.org/cl/676435.
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