Fix data race around static secret capability manager

[VioletHynes]

Description

Needed to copy the map behind the locking.

WARNING: DATA RACE
Write at 0x00c01a2f3470 by goroutine 86930:
  runtime.mapassign_faststr()
      /home/runner/actions-runner/_work/_tool/go/1.22.7/x64/src/runtime/map_faststr.go:203 +0x0
  github.com/hashicorp/vault/command/agentproxyshared/cache.(*LeaseCache).storeStaticSecretIndex()
      /home/runner/actions-runner/_work/vault-enterprise/vault-enterprise/command/agentproxyshared/cache/lease_cache.go:801 +0x5e4
  github.com/hashicorp/vault/command/agentproxyshared/cache.(*LeaseCache).cacheStaticSecret()
      /home/runner/actions-runner/_work/vault-enterprise/vault-enterprise/command/agentproxyshared/cache/lease_cache.go:696 +0x145e
  github.com/hashicorp/vault/command/agentproxyshared/cache.(*LeaseCache).Send()
      /home/runner/actions-runner/_work/vault-enterprise/vault-enterprise/command/agentproxyshared/cache/lease_cache.go:502 +0x22d6
  github.com/hashicorp/vault/command.(*ProxyCommand).Run.ProxyHandler.func13()
      /home/runner/actions-runner/_work/vault-enterprise/vault-enterprise/command/agentproxyshared/cache/handler.go:63 +0x5e5
  net/http.HandlerFunc.ServeHTTP()
      /home/runner/actions-runner/_work/_tool/go/1.22.7/x64/src/net/http/server.go:2171 +0x47
  net/http.(*ServeMux).ServeHTTP()
      /home/runner/actions-runner/_work/_tool/go/1.22.7/x64/src/net/http/server.go:2688 +0x1ef
  net/http.serverHandler.ServeHTTP()
      /home/runner/actions-runner/_work/_tool/go/1.22.7/x64/src/net/http/server.go:3142 +0x2a1
  net/http.(*conn).serve()
      /home/runner/actions-runner/_work/_tool/go/1.22.7/x64/src/net/http/server.go:2044 +0x13c4
  net/http.(*Server).Serve.gowrap3()
      /home/runner/actions-runner/_work/_tool/go/1.22.7/x64/src/net/http/server.go:3290 +0x4f

Previous read at 0x00c01a2f3470 by goroutine 87090:
  golang.org/x/exp/maps.Keys[go.shape.map[string]struct {},go.shape.string,go.shape.struct {}]()
      /home/runner/go/pkg/mod/golang.org/x/[email protected]/maps/maps.go:11 +0x27b
  github.com/hashicorp/vault/command/agentproxyshared/cache.(*StaticSecretCapabilityManager).StartRenewingCapabilities.func1()
      /home/runner/actions-runner/_work/vault-enterprise/vault-enterprise/command/agentproxyshared/cache/static_secret_capability_manager.go:148 +0x305
  github.com/gammazero/workerpool.worker()
      /home/runner/go/pkg/mod/github.com/gammazero/[email protected]/workerpool.go:237 +0x35
  github.com/gammazero/workerpool.(*WorkerPool).dispatch.gowrap2()
      /home/runner/go/pkg/mod/github.com/gammazero/[email protected]/workerpool.go:197 +0x4f

TODO only if you're a HashiCorp employee

• Backport Labels: If this PR is in the ENT repo and needs to be backported, backport
  to N, N-1, and N-2, using the backport/ent/x.x.x+ent labels. If this PR is in the CE repo, you should only backport to N, using the backport/x.x.x label, not the enterprise labels.
  • If this fixes a critical security vulnerability or severity 1 bug, it will also need to be backported to the current LTS versions of Vault. To ensure this, use all available enterprise labels.
• ENT Breakage: If this PR either 1) removes a public function OR 2) changes the signature
  of a public function, even if that change is in a CE file, double check that
  applying the patch for this PR to the ENT repo and running tests doesn't
  break any tests. Sometimes ENT only tests rely on public functions in CE
  files.
• Jira: If this change has an associated Jira, it's referenced either
  in the PR description, commit message, or branch name.
• RFC: If this change has an associated RFC, please link it in the description.
• ENT PR: If this change has an associated ENT PR, please link it in the
  description. Also, make sure the changelog is in this PR, not in your ENT PR.

[github-actions]

CI Results:
All Go tests succeeded! ✅

[github-actions]

Build Results:
All builds succeeded! ✅