github-actions: use github secrets

[v1v]

Vault free.

Tests are failing:

   ⚡️ FAIL  Elastic.OpenTelemetry.EndToEndTests.EndToEndTests.LatencyShowsAGraph
  
  Error: System.TimeoutException : Timeout 30000ms exceeded.
  Call log:
    - waiting for GetByRole(AriaRole.Heading, new() { Name = "Services" }) to be visible
  
    Failed Elastic.OpenTelemetry.EndToEndTests.EndToEndTests.LatencyShowsAGraph [2 s]
    Error Message:
     System.TimeoutException : Timeout 30000ms exceeded.
  Call log:
    - waiting for GetByRole(AriaRole.Heading, new() { Name = "Services" }) to be visible
    Stack Trace:
       at Microsoft.Playwright.Transport.Connection.InnerSendMessageToServerAsync[T](ChannelOwner object, String method, Dictionary`2 dictionary, Boolean keepNulls) in /_/src/Playwright/Transport/Connection.cs:line 207
     at Microsoft.Playwright.Transport.Connection.WrapApiCallAsync[T](Func`1 action, Boolean isInternal) in /_/src/Playwright/Transport/Connection.cs:line 537
     at Elastic.OpenTelemetry.EndToEndTests.DistributedFixture.ApmUIBrowserContext.WaitForServiceOnOverview(IPage page) in /home/runner/work/elastic-otel-dotnet/elastic-otel-dotnet/tests/Elastic.OpenTelemetry.EndToEndTests/DistributedFixture/ApmUIBrowserContext.cs:line 95
     at Elastic.OpenTelemetry.EndToEndTests.DistributedFixture.ApmUIBrowserContext.InitializeAsync() in /home/runner/work/elastic-otel-dotnet/elastic-otel-dotnet/tests/Elastic.OpenTelemetry.EndToEndTests/DistributedFixture/ApmUIBrowserContext.cs:line 50
     at Elastic.OpenTelemetry.EndToEndTests.DistributedFixture.ApmUIBrowserContext.InitializeAsync() in /home/runner/work/elastic-otel-dotnet/elastic-otel-dotnet/tests/Elastic.OpenTelemetry.EndToEndTests/DistributedFixture/ApmUIBrowserContext.cs:line 60
     at Elastic.OpenTelemetry.EndToEndTests.DistributedFixture.DistributedApplicationFixture.InitializeAsync() in /home/runner/work/elastic-otel-dotnet/elastic-otel-dotnet/tests/Elastic.OpenTelemetry.EndToEndTests/DistributedFixture/DistributedApplicationFixture.cs:line 114
     at Nullean.Xunit.Partitions.Sdk.PartitionTestAssemblyRunner.UseStateAndRun(IPartitionLifetime partition, Func`2 runGroup, Func`3 failAll)
    Standard Output Messages:
   Timeout 30000ms exceeded.
   Call log:
     - waiting for GetByRole(AriaRole.Heading, new() { Name = "Services" }) to be visible

Similarly happening in main:

• https://github.com/elastic/elastic-otel-dotnet/actions/runs/9578099798