Fix flaky TestCredSetup with -race by removing CI_DEBUG_TRACE

What does this MR do?

When running TestCredSetup with the -race flag, the test would intermittently fail with an assertion error indicating that the expected git insteadOf configuration was not found in the output.

The root cause was that CI_DEBUG_TRACE=1 enables shell tracing (set -o xtrace in bash), which causes all commands to be traced. With the race detector's slower execution and different buffering behavior, the trace output from the script's final "exit 0" command would interleave with the git config output, corrupting lines like:

    url.https://gitlab-ci-token:[MASKED]+ exit 0

where "+ exit 0" is bash's trace output bleeding into the git config listing.

The fix simply removes CI_DEBUG_TRACE=1 from the test, as it's not necessary for validating credential setup.

For example, in https://gitlab.com/gitlab-org/gitlab-runner/-/jobs/12342190999:

The fix explicitly disables shell tracing (set +x) before running git config -l in the bash case. This ensures that even if tracing is enabled globally in the test environment, it won't corrupt the git config output that the test needs to parse and validate.

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