Replace static, unsweepable name in TestAccCloudBuildTrigger_pubsub_config

[SarahFrench]

Addresses this test failure I saw in the nightly tests:

TestAccCloudBuildTrigger_pubsub_config


Stacktrace
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------- Stdout: -------
=== RUN   TestAccCloudBuildTrigger_pubsub_config
=== PAUSE TestAccCloudBuildTrigger_pubsub_config
=== CONT  TestAccCloudBuildTrigger_pubsub_config
    vcr_utils.go:152: Step 1/4 error: Error running apply: exit status 1
        Error: Error creating Topic: googleapi: Error 409: Resource already exists in the project (resource=topic-name).
          with google_pubsub_topic.build-trigger,
          on terraform_plugin_test.tf line 2, in resource "google_pubsub_topic" "build-trigger":
           2: resource "google_pubsub_topic" "build-trigger" {
--- FAIL: TestAccCloudBuildTrigger_pubsub_config (6.37s)
FAIL

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read Write release notes before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 1 file changed, 4 insertions(+), 4 deletions(-))
Terraform Beta: Diff ( 1 file changed, 4 insertions(+), 4 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2939
Passed tests 2636
Skipped tests: 302
Affected tests: 1

Action taken

Found 1 affected test(s) by replaying old test recordings. Starting RECORDING based on the most recent commit. Click here to see the affected tests

TestAccCloudBuildTrigger_pubsub_config

Get to know how VCR tests work

[modular-magician]

$\textcolor{green}{\textsf{Tests passed during RECORDING mode:}}$
TestAccCloudBuildTrigger_pubsub_config[Debug log]

Rerun these tests in REPLAYING mode to catch issues

$\textcolor{green}{\textsf{No issues found for passed tests after REPLAYING rerun.}}$

---

$\textcolor{green}{\textsf{All tests passed!}}$
View the build log or the debug log for each test