fix(systemendpoint): prevent data race on server

[tessig]

We must expect shutdown to be called several times and therefore lock the process.

[github-actions]

Package	Line Rate	Health
flamingo.me/flamingo/v3/core/auth	13%	❌
flamingo.me/flamingo/v3/core/auth/fake	50%	➖
flamingo.me/flamingo/v3/core/auth/http	40%	❌
flamingo.me/flamingo/v3/core/auth/oauth	48%	❌
flamingo.me/flamingo/v3/core/cache	54%	➖
flamingo.me/flamingo/v3/core/gotemplate	87%	✔
flamingo.me/flamingo/v3/core/healthcheck/interfaces/controllers	76%	✔
flamingo.me/flamingo/v3/core/healthcheck	64%	➖
flamingo.me/flamingo/v3/core/internalauth/application	100%	✔
flamingo.me/flamingo/v3/core/internalauth	100%	✔
flamingo.me/flamingo/v3/core/locale/application	87%	✔
flamingo.me/flamingo/v3/core/locale/domain	75%	✔
flamingo.me/flamingo/v3/core/locale/interfaces/controllers	100%	✔
flamingo.me/flamingo/v3/core/locale/interfaces/templatefunctions	92%	✔
flamingo.me/flamingo/v3/core/locale	59%	➖
flamingo.me/flamingo/v3/core/oauth/application	4%	❌
flamingo.me/flamingo/v3/core/oauth/domain	62%	➖
flamingo.me/flamingo/v3/core/oauth	55%	➖
flamingo.me/flamingo/v3/core/requestlogger	100%	✔
flamingo.me/flamingo/v3/core/requesttask	10%	❌
flamingo.me/flamingo/v3/core/robotstxt	50%	➖
flamingo.me/flamingo/v3/core/runtime	77%	✔
flamingo.me/flamingo/v3/core/security/application/role	95%	✔
flamingo.me/flamingo/v3/core/security/application	98%	✔
flamingo.me/flamingo/v3/core/security/application/voter	100%	✔
flamingo.me/flamingo/v3/core/security/interface/controller	100%	✔
flamingo.me/flamingo/v3/core/security/interface/middleware	72%	➖
flamingo.me/flamingo/v3/core/security	59%	➖
flamingo.me/flamingo/v3/core/zap	43%	❌
flamingo.me/flamingo/v3/framework/cmd	12%	❌
flamingo.me/flamingo/v3/framework/config	49%	❌
flamingo.me/flamingo/v3/framework/flamingo	30%	❌
flamingo.me/flamingo/v3/framework	29%	❌
flamingo.me/flamingo/v3/framework/opencensus	88%	✔
flamingo.me/flamingo/v3/framework/prefixrouter	43%	❌
flamingo.me/flamingo/v3/framework/systemendpoint	27%	❌
flamingo.me/flamingo/v3/framework/testutil	0%	❌
flamingo.me/flamingo/v3/framework/web	49%	❌
Summary	49% (3278 / 6678)	❌