ci: separate slather step

[armcknight]

I noticed that sometimes unit test jobs still report failures even though no tests fail, with the script exiting nonzero. See https://github.com/getsentry/sentry-cocoa/actions/runs/16505144160/job/46673775222

I thought maybe this is actually an error coming from slather. Separate that into its own step so we can isolate potential failures.

#skip-changelog

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
27833	1	27832	85

View the top 1 failed test(s) by shortest run time

SentryTracerTests::testConcurrentTransactions_OnlyOneGetsMeasurement

Stack Traces | 0s run time

Restarting after unexpected exit, crash, or test timeout in SentryTracerTests.testConcurrentTransactions_OnlyOneGetsMeasurement(); summary will include totals from previous launches.

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[philipphofmann]

Good catch thanks

[itaybre]

Looks like this still failed with the same error here.
Might be related to the teardown of xcodebuild, not the slather step