cloud_roles: fix unused return value warning

[dotnwat]

/home/nwatkins/src/redpanda-clang-18/src/v/cloud_roles/tests/credential_tests.cc:89:9: error: the value returned by this function should not be disregarded; neglecting it may lead to errors [bugprone-unused-return-value,-warnings-as-errors]
   89 |         applier.add_auth(h);
      |         ^~~~~~~~~~~~~~~~~~~
/home/nwatkins/src/redpanda-clang-18/src/v/cloud_roles/tests/credential_tests.cc:99:9: error: the value returned by this function should not be disregarded; neglecting it may lead to errors [bugprone-unused-return-value,-warnings-as-errors]
   99 |         applier.add_auth(h);
      |  
       ^~~~~~~~~~~~~~~~~~~

Backports Required

• [x] none - not a bug fix
• [ ] none - this is a backport
• [ ] none - issue does not exist in previous branches
• [ ] none - papercut/not impactful enough to backport
• [ ] v24.2.x
• [ ] v24.1.x
• [ ] v23.3.x

Release Notes

• none

[dotnwat]

only affected unit tests, and those passed.

[dotnwat]

nice q (for my own benefit): did the default behavior of bugprone-unused-return-value change or are we changing the config?

@oleiman it seems that clang-tidy-18 caught this while clang-tidy-16 did not.