ci: Bump tsan config to ubuntu:26.04 with -U_FORTIFY_SOURCE #35609

The -U_FORTIFY_SOURCE should be harmless in the Tsan CI, and is required to work around https://github.com/bitcoin/bitcoin/issues/30586

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35609.

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🚧 At least one of the CI tasks failed. _{Task lint: https://github.com/bitcoin/bitcoin/actions/runs/28255074722/job/83715952341} _{LLM reason (✨ experimental): CI failed because the shell lint (ShellCheck SC2155) reported a warning in ci/test/00_setup_env_native_tsan.sh line 19 (in lint-shell.py), causing the lint job to exit non-zero.}

<details><summary>Hints</summary>

Try to run the tests locally, according to the documentation. However, a CI failure may still happen due to a number of reasons, for example:

• Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.

• An intermittent issue.

Leave a comment here, if you need help tracking down a confusing failure.

</details>

ACK bbbbab86a81e137e83fbaed4ec1ba7318c7e8a11