fix(longevity-alternator-3h): set different default AZ

since this test is running on `i4i.4xlarge` by default,
and the availability for these instances on AZ `a` is
minimal, as the following error shows:
```
(InsufficientInstanceCapacity) when calling the RunInstances
operation (reached max retries: 4): We currently do not have
sufficient i4i.4xlarge capacity in the Availability Zone you
requested (us-east-1a). Our system will be working on
provisioning additional capacity. You can currently get
i4i.4xlarge capacity by not specifying an Availability Zone
in your request or choosing us-east-1b, us-east-1c,
us-east-1d, us-east-1f.
```

moving the default to `c` did make the 5.4 job at least start,
so sending this PR to change the default, to increase the
chance of success.

jenkins-pipelines/longevity-alternator-3h.jenkinsfile

@@ -6,6 +6,7 @@ def lib = library identifier: 'sct@snapshot', retriever: legacySCM(scm)
 longevityPipeline(
     backend: 'aws',
     region: 'eu-west-1',
+    availability_zone: 'c',
     test_name: 'longevity_test.LongevityTest.test_custom_time',
     test_config: 'test-cases/longevity/longevity-alternator-3h.yaml',