build: cosign retry resilience — add jitter and raise max-attempts default

[bedatty]

Split from #421 (Fix 1 resilience items). The 2.2.0-beta.6 incident's attempt #1 failed because cosign signing hit a transient Rekor 404 getLogEntryByUuidNotFound across all 3 retry attempts. Rekor intermittency can last several minutes, so the current retry policy is insufficient.

Current state (develop)

• build.yml inputs: cosign_max_attempts default 3, cosign_initial_delay default 5.
• src/security/cosign-sign/action.yml: exponential backoff (delay ×3 per failed attempt), no jitter.

Proposed

• Add jitter to the cosign retry delay (randomized component) to avoid thundering-herd when multiple jobs hit Rekor simultaneously.
• Review/raise the cosign_max_attempts default (e.g. 3 → 5) and consider a higher backoff ceiling, to ride out multi-minute Rekor outages.

Scope notes

• Touches src/security/cosign-sign/action.yml (retry loop) and possibly the build.yml defaults.
• The template-error that broke continue_gitops_on_signing_failure during this same incident is fixed separately (see the PR for build: pre-flight tag existence check + cleanup on downstream failure #421's attempt-ci(deps): bump actions/setup-node from 4 to 5 #1/ci(deps): bump actions/checkout from 4 to 5 #2 fixes).

Related: #421