fix: [TKC-3433] fix fatal error: concurrent map writes

[povilasv]

Pull request description

Adds read write mutex to protect the cache, our cloud-api is currently failing with:

{"level":"warn","ts":"2025-03-20T11:34:51Z","caller":"agentserver/agent_stream.go:40","msg":"error getting environment by api key","version":"1.11.18","build":"cloud","service":"AgentServer","error":"record does not exists","errorVerbose":"record does not exists:\n    github.com/kubeshop/testkube-cloud-api/pkg/repository/common.init\n        /app/pkg/repository/common/errors.go:5","obfuscatedKey":"tkcagnt_02**************************bf"}
fatal error: concurrent map writes

goroutine 3400656 [running]:
github.com/kubeshop/testkube/pkg/testworkflows/testworkflowexecutor.(*testWorkflowTemplateFetcher).Prefetch(0xc001cfb830, {0xc004ebd904?, 0x0?})
	/root/.cache/go-build/github.com/kubeshop/[email protected]/pkg/testworkflows/testworkflowexecutor/testworkflowtemplatefetcher.go:53 +0xec
github.com/kubeshop/testkube/pkg/testworkflows/testworkflowexecutor.(*testWorkflowTemplateFetcher).PrefetchMany.(*testWorkflowTemplateFetcher).PrefetchMany.func1.func2()
	/root/.cache/go-build/github.com/kubeshop/[email protected]/pkg/testworkflows/testworkflowexecutor/testworkflowtemplatefetcher.go:70 +0x1f
golang.org/x/sync/errgroup.(*Group).Go.func1()
	/root/.cache/go-build/golang.org/x/[email protected]/errgroup/errgroup.go:78 +0x50
created by golang.org/x/sync/errgroup.(*Group).Go in goroutine 3400649
	/root/.cache/go-build/golang.org/x/[email protected]/errgroup/errgroup.go:75 +0x96

Checklist (choose whats happened)

• breaking change! (describe)
• tested locally
• tested on cluster
• added new dependencies
• updated the docs
• added a test

Breaking changes

Changes

Fixes