`WithEnvironment` does not override actual environment variable in runtime

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

Setting the OTEL endpoint via WithEnvironment is reflected in the Aspire Dashboard, but the application ignores it at runtime. For example:

`builder.AddProject("weather-api")
.WithEnvironment("OTEL_EXPORTER_OTLP_ENDPOINT","http://localhost:4317");`

Dashboard shows http://localhost:4317 for `OTEL_EXPORTER_OTLP_ENDPOINT`

Runtime actually uses a different endpoint like http://localhost:62312

This prevents redirecting traces to a custom OTEL collector.

Notes:

I have not called AddOpenTelemetryCollector(...), per previous feedback (#11298) that WithEnvironment should suffice. It's not needed to reproduce the problem. However if used, the behaviour is exactly the same.

The discrepancy is between what Aspire reports and the actual environment in the running app.

Repo for reproduction: [AspireOtelDemo](https://github.com/kcadduk/AspireOtelDemo)

### Expected Behavior

WithEnvironment should respect the configured OTEL endpoint

### Steps To Reproduce

As Above

### Exceptions (if any)

None

### .NET Version info

9 & 10

### Anything else?

None