Do not run performance benchmarks for tags

[adpaco-aws]

Description of changes:

Currently, the performance benchmarks also run when pushing kani-* tags, which is essentially done each time we're going to publish a new release. In that case, the performance CI job fails with the error:

Initializing the repository
Disabling automatic garbage collection
Setting up auth
Fetching the repository
  /usr/bin/git -c protocol.version=2 fetch --no-tags --prune --progress --no-recurse-submodules --depth=2 origin 0000000000000000000000000000000000000000
  Error: fatal: remote error: upload-pack: not our ref 0000000000000000000000000000000000000000
  The process '/usr/bin/git' failed with exit code 128
  Waiting 17 seconds before trying again

There's at least two occasions in which I've seen this happen:

• For the kani-0.35.0 tag: https://github.com/model-checking/kani/actions/runs/5957239259/job/16159564100
• For the kani-0.36.0 tag: https://github.com/model-checking/kani/actions/runs/6110889802/job/16585358347

It doesn't make sense to run performance benchmarks in this case, since the candidate commit will have already triggered a performance benchmarks CI job when it's merged into main.

Resolved issues:

Resolves #2744

Testing:

• How is this change tested? N/A (changes to GA workflows cannot be tested)

• Is this a refactor change? No.

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.