os: update examples to use errors.Is instead of deprecated helpers

[kovan]

The os.IsExist and os.IsNotExist functions predate errors.Is
and their documentation recommends using
errors.Is(err, fs.ErrExist) and errors.Is(err, fs.ErrNotExist)
in new code.

Update ExampleMkdir and ExampleUserConfigDir to follow this
recommendation.

Fixes #77643

[gopherbot]

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[gopherbot]

Message from Sean Liao:

Patch Set 1: Auto-Submit+1 Code-Review+2 Commit-Queue+1

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Message from Go LUCI:

Patch Set 1:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2026-04-03T01:06:18Z","revision":"8b4ff570cddfcf2dc9269fab4c043137865882f7"}

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Message from Sean Liao:

Patch Set 1: -Commit-Queue

(Performed by <GERRIT_ACCOUNT_60063> on behalf of <GERRIT_ACCOUNT_34788>)

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Message from Go LUCI:

Patch Set 1:

This CL has failed the run. Reason:

Tryjob golang/try/gotip-linux-386_debiansid has failed with summary (view all results):

---

• go on master (change 762400)

To reproduce, try gomote repro 8685584265789393825.

Additional links for debugging:

• go tool dist test -json output [shard 2]

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Message from Go LUCI:

Patch Set 1: LUCI-TryBot-Result-1

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